linux-user: prefer glibc over direct syscalls

linux-user: prefer glibc over direct syscalls

The openat/*at syscalls are incredibly common with modern coreutils,
calling them directly via syscalls breaks for example fakeroot. Use
glibc stubs whenever directly available and provide old syscall
calling for people still using older libc.

Patch originally from Mika Westerberg, Adapted to
apply to current trunk and cleaned up by Riku Voipio.

Signed-off-by: Riku Voipio <riku.voipio@iki.fi>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@7118 c046a42c-6fe2-441c-8c8c-71466251a162