Also remove unneeded platform gating

Rebase of #610 and also move emscripten up much higher in the hierarchy to
ensure that it doesn't have too much of a ripple effect on other platforms.

This involved moving down a good number of definitions, but hopefully was done
with care to not break anything!

We now create an additional binary `linux_fcntl` for testing this
since there are header conflicts when including all necessary headers.
This binary is run on all platforms even though it's empty on all non-
Android/non-Linux platforms.

Testing has been switched from a custom binary to using a runner-less
test (or pair of tests). This means that for local development a simple
`cd libc-test && cargo test` will run all the tests. CI has also been
updated here to reflect that.

It's now broken due to changes in the `gcc` crate and having a too-old compiler,
and in general it's unfortunately architecturally so different from the other
test frameworks that it's difficult to maintain over time.

they will be released with OpenBSD 6.2

the type changed from *c_char to *c_void in 6.1

Add missing fexecve() - execute program specified via file descriptor.

Reference: http://pubs.opengroup.org/onlinepubs/9699919799/functions/fexecve.html

glibc support is behind, as indicated by
https://github.com/karelzak/util-linux/blob/master/sys-utils/fallocate.c

It needs to be skipped during testing because there's no way to include
a combination of headers that will expose it without giving a "type has
already been defined" error.

Make changes to reflect this

vfork, wait4, getresuid, getresgid, personality, vhangup,
swapon, swapoff, getdomainname, setdomainname, iopl, ioperm

Fixed the issues that prevented the libc_tests that are generated from passing. Also, fixed struct names and types to be representative of the linux source files.

_GNU_SOURCE works differently than other libraries and creates
function pointer issues

The default  on OS X does not provide clear errors,
instead the NOCANCEL variants should be used. These are available
as of OS X 10.6 onwards.

There is a compatibility issue regarding the type of a flag parameter:
int vs uint. Linux does declare the syscall to use uint and musl
followed it, but it is incompatible with other POSIX recv*/send*
syscalls. So it seems to be wise to follow the glibc version of
prototypes with int.

Signed-off-by: NODA, Kai <nodakai@gmail.com>