linux: Optimize fallback 32-bit clock_getres

This patch avoid probing the __NR_clock_getttime64 syscall each time
__clock_gettime64 is issued on a kernel without 64 bit time support.
Once ENOSYS is obtained, only 32-bit clock_gettime are used.

The following snippet:

  clock_getres (CLOCK_REALTIME, &(struct timespec) { 0 });
  clock_getres (CLOCK_MONOTONIC, &(struct timespec) { 0 });
  clock_getres (CLOCK_BOOTTIME, &(struct timespec) { 0 });
  clock_getres (20, &(struct timespec) { 0 });

On a kernel without 64 bit time support issues the syscalls:

  syscall_0x196(0, 0xffb83330, [...]) = -1 ENOSYS (Function not implemented)
  clock_getres(CLOCK_REALTIME, {tv_sec=0, tv_nsec=1}) = 0
  clock_getres(CLOCK_MONOTONIC, {tv_sec=0, tv_nsec=1}) = 0
  clock_getres(CLOCK_BOOTTIME, {tv_sec=0, tv_nsec=1}) = 0

Checked on i686-linux-gnu on 4.15 kernel.

Reviewed-by: Siddhesh Poyarekar <siddhesh@sourceware.org>