timer-linux: fallback to CLOCK_MONOTONIC instead of timespec_get

CLOCK_MONOTONIC_RAW is linux-specific (macOS later supported it but it
has its own timer code) and not neccessarily available everywhere like
on BSDs. It makes sense to prefer it because mpv does a lot of
measurements at small intervals (e.g. every frame) so theoretically it
should be more accurate. However if the OS doesn't have it, fallback to
CLOCK_MONOTONIC instead which is almost exactly the same and very widely
supported across unix-like systems. This clock is technically optional
according to POSIX, but any half-decent OS supports it anyway (sorry
Solaris users). As a benefit, we now know that the clock from mp_time is
always monotonic.

osdep/timer-linux.c

@@ -35,11 +35,12 @@ void mp_sleep_ns(int64_t ns)
 uint64_t mp_raw_time_ns(void)
 {
     struct timespec tp = {0};
+    int ret = -1;
 #if defined(CLOCK_MONOTONIC_RAW)
-    clock_gettime(CLOCK_MONOTONIC_RAW, &tp);
-#else
-    timespec_get(&tp, TIME_UTC);
+    ret = clock_gettime(CLOCK_MONOTONIC_RAW, &tp);
 #endif
+    if (ret)
+        clock_gettime(CLOCK_MONOTONIC, &tp);
     return MP_TIME_S_TO_NS(tp.tv_sec) + tp.tv_nsec;
 }