Move the link line for `libdl` up to `src/unix/mod.rs`, making it easier
to see all the libraries `libc` links to.

This also makes `libdl` respect `target-feature=+crt-static`.

The two library blocks that specify `#[link(name = "util")]` do not
actually reference any functions in `libutil`; the functions that do use
`libutil` don't have any reference to it. And having two library blocks
specify it results in two separate inclusions of `-lutil` on the linker
command line. Move the link lines up to `src/unix/mod.rs`, making it
easier to see all the libraries `libc` links to.

This also makes `libutil` respect `target-feature=+crt-static`.

This will need corresponding changes in rust-lang/rust to activate, but
this will make it possible to make those changes.

Note that despite the apparent redundancy in config directives, the link
directives cannot be simplified any further. Attempting to factor out
the checks for `target_feature = "crt-static"` does not work.

Skip test on aarch64 due to https://github.com/rust-lang/libc/issues/1765

DevkitPPC does not support unix sockets natively, meaning that bindings
to these functions was removed for powerpc targets with "nintendo" as
vendor.

Suggested target json files:

Nintendo Gamecube:
```
{
  "arch": "powerpc",
  "data-layout": "E-m:e-p:32:32-i64:64-n32",
  "dynamic-linking": false,
  "env": "newlib",
  "executables": true,
  "has-elf-tls": false,
  "has-rpath": true,
  "linker-flavor": "gcc",
  "llvm-target": "powerpc-eabi",
  "max-atomic-width": 32,
  "os": "dolphin",
  "target-c-int-width": "32",
  "target-endian": "big",
  "target-family": "unix",
  "target-mcount": "_mcount",
  "target-pointer-width": "32",
  "vendor": "nintendo"
}
```

Nintendo Wii:
```
{
  "arch": "powerpc",
  "data-layout": "E-m:e-p:32:32-i64:64-n32",
  "dynamic-linking": false,
  "env": "newlib",
  "executables": true,
  "has-elf-tls": false,
  "has-rpath": true,
  "linker-flavor": "gcc",
  "llvm-target": "powerpc-eabi",
  "max-atomic-width": 32,
  "os": "revolution",
  "target-c-int-width": "32",
  "target-endian": "big",
  "target-family": "unix",
  "target-mcount": "_mcount",
  "target-pointer-width": "32",
  "vendor": "nintendo"
}
```

The new ARM-based platform doesn't need these as there's no legacy
constraints.

Tested via

**demo.c**

```c

int main() {
  fstat(0, NULL);
  fstatat(0, NULL, NULL, 0);
  lstat(NULL, NULL);
  stat(NULL, NULL);
  readdir(NULL);
  readdir_r(NULL, NULL, NULL);
}
```

**Compilation**

```none
% SDKROOT=/Applications/Xcode-beta.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX11.0.sdk MACOSX_DEPLOYMENT_TARGET=11.5 cc -arch arm64 demo.c

% nm a.out
0000000100008030 d __dyld_private
0000000100000000 T __mh_execute_header
                 U _fstat
                 U _fstatat
                 U _lstat
0000000100003e64 T _main
                 U _readdir
                 U _readdir_r
                 U _stat
                 U dyld_stub_binder
```

This has also been experimentally compiled on a Developer Transition Kit.

Add strndup(), as specified by POSIX.1-2008.

This does not add sys_siglist because the docs specify that the function
should be used instead, whenever possible.

Fix #1631

This adds libc-test support for Freebsd10 and a CI build job that tests
FreeBSD10 with LIBC_CI only.

Currently, libc supports and detects freebsd11 and freebsd13
Unknown versions, like freebsd13, is treated as freebsd11.
This patch solve the issues, detecting freebsd13 and treating it like
freebsd12.
Inverting the logic not(freebsd12) -> freebsd11 where possible

Currently, libc supports and detects freebsd11 and freebsd13
Unknown versions, like freebsd13, is treated as freebsd11.
This patch solve the issues, detecting freebsd13 and treating it like
freebsd12.
Inverting the logic not(freebsd12) -> freebsd11 where possible

Add cfg_attr

This commits adds a second FreeBSD 12 build job,
and splits the implementation of the FreeBSD module
into two modules, one for FreeBSD 11, and one for FreeBSD 12.

The FreeBSD 11 module is compiled always by default, and is
mostly forward compatible with FreeBSD 12 systems.

The FreeBSD 12 module is only built for now in libc's CI,
and uses FreeBSD 12 data types and APIs, linking to symbols
that are only available in FreeBSD 12.

Basically, when LIBC_CI env variable is defined, and the host
system is a FreeBSD 12 system, then the FreeBSD 12 module is
automatically built and tested. Conditional compilation is done
using a `cfg(freebsd12)` flag.

This commit also re-enables many tests, and documents why
some remain disabled.

Closes #1055.

Related to #657 .

The second argument of `gettimeofday` was a `*mut c_void` on all targets,
but that type is incorrect in the following targets, where it should be
a `*mut timezone` instead:

On these other targets it appears that the signature of gettimeofday was incorrect (it takes a time-zone pointer instead of a void pointer):

linux+gnu: http://man7.org/linux/man-pages/man2/gettimeofday.2.html
freebsd: https://www.freebsd.org/cgi/man.cgi?query=gettimeofday&apropos=0&sektion=2&manpath=FreeBSD+11.2-stable&arch=default&format=html
openbsd: https://man.openbsd.org/gettimeofday.2
android: https://github.com/ricardoquesada/android-ndk/blob/master/usr/include/sys/time.h
dragonfly: https://www.dragonflybsd.org/cgi/web-man?command=gettimeofday&section=2

This commit corrects the type on these targets, which is a breaking change. Due
to how this API is commonly used (e.g. passing `ptr::null_mut` to the second
argument), breakage should be minimal. Users wanting to support both versions
can just write `ptr as *mut _` instead.

Closes #1338.

See https://github.com/rust-lang/rust/pull/60775 .