rvtimer: Use faster clocksources where possible

- On FreeBSD, there is CLOCK_MONOTONIC_FAST which is much faster than CLOCK_MONOTONIC but more coarse
- On Linux, there is CLOCK_MONOTONIC_COARSE

src/rvtimer.c

@@ -56,9 +56,13 @@ uint64_t rvtimer_clocksource(uint64_t freq)
 }
 
 #elif defined(CLOCK_REALTIME) || defined(CLOCK_MONOTONIC)
-// Use POSIX clock_gettime(), with a monotonic clock if possible
+// Use POSIX clock_gettime(), with a fast monotonic clock if possible
 #include <unistd.h>
-#if defined(CLOCK_MONOTONIC_RAW)
+#if defined(CLOCK_MONOTONIC_FAST)
+#define CHOSEN_POSIX_CLOCK CLOCK_MONOTONIC_FAST
+#elif defined(CLOCK_MONOTONIC_COARSE)
+#define CHOSEN_POSIX_CLOCK CLOCK_MONOTONIC_COARSE
+#elif defined(CLOCK_MONOTONIC_RAW)
 #define CHOSEN_POSIX_CLOCK CLOCK_MONOTONIC_RAW
 #elif defined(CLOCK_MONOTONIC)
 #define CHOSEN_POSIX_CLOCK CLOCK_MONOTONIC