libc: Fix build on case-insensitive file systems

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()")
Reviewed by: kib
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D31895