armstdc: Fixup headers as needed by posix

* Move 'struct _timespec' into sys/_timespec.h as expected by
  <posix/time.h>.

* Introduce 'struct timeval' in sys/_timeval.h as expected by
  <posix/time.h>.

* Add mode_t to <sys/types.h> as expected by <posix/stat.h>

Signed-off-by: Kumar Gala <kumar.gala@intel.com>