linux-user: Fix sys_utimensat (would not compile on old glibc)

Commit c0d472b12e accidentally dropped the definition of
__NR_SYS_utimensat even though its use is guarded by
CONFIG_UTIMENSAT, not CONFIG_ATFILE. Some older glibc don't
have utimensat() (even if they have the other *at() functions).
Fix this by correctly cleaning up the sys_utimensat()
implementation and #defines, so that we always provide the
syscall if needed whether we're doing it via glibc or not.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Tested-by: Laurent Desnogues <laurent.desnogues@gmail.com>
Message-id: 1371743841-26110-1-git-send-email-peter.maydell@linaro.org
