linux-user: support timerfd_{create, gettime, settime} syscalls

Adds support for the timerfd_create, timerfd_gettime & timerfd_settime
syscalls, allowing use of timerfds by target programs.

v2: By Riku - added configure check for timerfd and ifdefs
for benefit of old distributions like RHEL5.

Signed-off-by: Paul Burton <paul@archlinuxmips.org>
Signed-off-by: Riku Voipio <riku.voipio@linaro.org>
