mach_clock: Fix monotonic clock sometimes going backwards

Between reading mtime and reading hpclock_read_counter,
there may be an interrupt that updates mtime, therefore
we need a check to perform the clock read process again
in this case.

TESTED: on UP using:

```
\#include <stdio.h>
\#include <time.h>

int main()
{
  struct timespec ts, now;
  int i;
  int cnt = 0;

  clock_gettime(CLOCK_MONOTONIC, &ts);

  for (i = 0; i < 10000000; i++) {
    clock_gettime(CLOCK_MONOTONIC, &now);
    if ((now.tv_nsec < ts.tv_nsec) && (now.tv_sec <= ts.tv_sec)) {
      printf("BACKWARDS\n");
      cnt++;
      printf("  %u %09lu\n  %u %09lu\n\n",
        ts.tv_sec, ts.tv_nsec,
        now.tv_sec, now.tv_nsec
      );
    }
    ts = now;
  }
  printf("went backwards %d out of %d times\n", cnt, i);
  return 0;
}
```

Before the change, some backward transitions were detected.
After this change, none were detected.
Message-ID: <20250921091345.2183347-1-damien@zamaudio.com>

0 files changed, 0 insertions, 0 deletions