libc: Fix build on case-insensitive file systems

[jrtc27]

Summary

On case-insensitive file systems (most likely to be seen on macOS, where
it is the default), _Fork.o for the new POSIX _Fork function conflicts
with _fork.o for the PSEUDO file. This results in non-determinsitic
behaviour in terms of which ends up being present; if _Fork.o wins then
the build fails to link libc.so due to missing __sys_fork, and if
_fork.o wins then libc silently fails to include the implementation of
_Fork. A similar issue occurred in the past for C99's _Exit conflicting
with exit(2) and was fixed in cb1cb6a2a83f, so this adds a fix based on
that.

As a longer-term solution it might be better to instead make the
generated files use a different prefix that's less likely to conflict
with other things (such as __sys_foo.o given they always contain that)
but that's a rather more invasive change.

Fixes: 49ad342cc10c ("Add _Fork()")
MFC after: 1 week

[kib]

Please just rename _Fork.c to something else, if it makes an issue. Next people to complain would be somebody without symlinks, like new and improved MacOSX users.

[jrtc27]

In D31895#719630, @kib wrote:

Please just rename _Fork.c to something else, if it makes an issue. Next people to complain would be somebody without symlinks, like new and improved MacOSX users.

Well you're SOL if your system doesn't support symlinks, that's an extremely hard requirement of our build... but sure I can do that, just figured it was best to copy an existing "solution"

[jrtc27]

Just rename the file rather than adding a symlink at build time