riscv: Fix building on non-Linux OS's

CLOCK_MONOTONIC_RAW is not POSIX/portable.

tests/checkasm/checkasm.h

@@ -208,7 +208,11 @@ static inline uint64_t readtime(void) {
 #include <time.h>
 static inline uint64_t clock_gettime_nsec(void) {
   struct timespec ts;
+#ifdef CLOCK_MONOTONIC_RAW
   clock_gettime(CLOCK_MONOTONIC_RAW, &ts);
+#else
+  clock_gettime(CLOCK_MONOTONIC, &ts);
+#endif
   return ((uint64_t)ts.tv_sec*1000000000u) + (uint64_t)ts.tv_nsec;
 }
 #define readtime clock_gettime_nsec