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.

Use safe_f! consistently across platforms

The pr #1870 introduced safe_f! macro, which made some functions like
WIFEXITED and WEXITSTATUS const and safe on linux_like platform only,
which causes inconsistency when trying to use those functions in crates
compiled across multiple platforms, as using unsafe on those functions
will generate unused_unsafe warning on linux platforms and lack of
unsafe block will fail compilation on non-linux platforms.

To avoid the inconsistency, this commit applies the same macro for all
the same functions on other platforms too.

Bump up to 0.2.77

add pthread_getattr_np() and pthread_attr_getstack() for Solaris

Define several more constants for WASI.

Add getitimer and setitimer for macOS

Define `O_NOCTTY`, `R_OK`, `POLLIN`, and related constants using the
current values from WASI libc.

Add getitimer and setitimer for macOS

Add missing syscall numbers for aarch64-musl

Add support for Apple's O_SYMLINK flag

Add IOV_MAX and UIO_MAXIOV constants

These constant can be used to determine the maximum number of iovecs can
be passed to functions like readv/writev.

Linux like uses UIO_MAXIOV, while the BSD family uses IOV_MAX.

Haiku: add [get/set]priority()

Expose __errno_location() (introduced in DragonFlyBSD 5.8), which
returns the current thread's errno value. This is similar to Linux
and avoids having a separate module that defines both errno (which
depends on the thread_local feature) and an __error() function.

These methods were added in Haiku R1 beta 2.

Update Windows Server and Visual Studio versions to 2019

Added clock_getcpuclockid to more targets

Previously clock_getcpuclockid was enabled only on linux, with this
change it is enabled on all linux_like and freebsdlike.

Clarify the use of external Docker image

Update emsdk to 1.39.20

Bump up to 0.2.76

Let's prevent this from happening again.

libc 0.2.75