linux-user: Drop direct use of openat etc syscalls

linux-user: Drop direct use of openat etc syscalls

The linux-user syscall emulation layer currently supports the
openat family of syscalls via two mechanisms: simply calling
the corresponding libc functions, and making direct syscalls.
Since glibc has supported these functions since at least glibc
2.5, there's no real need to retain the (essentially untested)
direct syscall fallback code, so simply delete it. This allows
us to remove some ifdeffery that was attempting to disable
provision of some of the syscalls if the host didn't seem to
support them, which in some cases was actually wrong (eg where
there are several flavours of the syscall and we only need
one of them, not necessarily the exact one the guest has,
as with the fstatat* calls).

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <rth@twiddle.net>
Tested-by: Claudio Fontana <claudio.fontana@huawei.com>
Message-id: 1370126121-22975-2-git-send-email-peter.maydell@linaro.org