Read System Clock for Monotonic Time

This PR changes the `janet_gettime` implementation for OSX <10.12 to
read the system clock for `(os/clock :monotonic)`. As far as I was able
to find online this is _a_ monotonic clock, although it produces
different values from `clock_gettime(CLOCK_MONOTONIC, ...)` on the same
system. I can speculate that this is related to `SYSTEM_CLOCK` monotonic
time being implemented with `mach_absolute_time` which is documented to
_not advance during sleep_, and I suspect that
`clock_gettime(CLOCK_MONOTONIC, ...)` does.

**Resources**:
- `clock_get_time` implementation for the `SYSTEM_CLOCK`:
  <https://github.com/apple-oss-distributions/xnu/blob/e3723e1f17661b24996789d8afc084c0c3303b26/osfmk/kern/clock_oldops.c#L284-L296>
  <https://github.com/apple/darwin-xnu/blob/2ff845c2e033bd0ff64b5b6aa6063a1f8f65aa32/osfmk/arm/rtclock.c#L248-L260>
- `mach_absolute_time` and `mach_continuous_time` definitions:
  <https://github.com/apple-oss-distributions/xnu/blob/e3723e1f17661b24996789d8afc084c0c3303b26/osfmk/mach/mach_time.h#L55-L68>
- Stack overflow post for implementing `clock_gettime` on OS X before 10.12:
  <https://stackoverflow.com/questions/11680461/monotonic-clock-on-osx>

Author: nlordell

src/core/util.c

@@ -931,27 +931,24 @@ int janet_gettime(struct timespec *spec, enum JanetTimeSource source) {
 #include <mach/clock.h>
 #include <mach/mach.h>
 int janet_gettime(struct timespec *spec, enum JanetTimeSource source) {
-    if (source == JANET_TIME_REALTIME) {
+    if (source == JANET_TIME_CPUTIME) {
+        clock_t tmp = clock();
+        spec->tv_sec = tmp / CLOCKS_PER_SEC;
+        spec->tv_nsec = ((tmp - (spec->tv_sec * CLOCKS_PER_SEC)) * 1000000000) / CLOCKS_PER_SEC;
+    } else {
         clock_serv_t cclock;
         mach_timespec_t mts;
-        host_get_clock_service(mach_host_self(), CALENDAR_CLOCK, &cclock);
+        clock_id_t cid = CALENDAR_CLOCK;
+        if (source == JANET_TIME_REALTIME) {
+            cid = CALENDAR_CLOCK;
+        } else if (source == JANET_TIME_MONOTONIC) {
+            cid = SYSTEM_CLOCK;
+        }
+        host_get_clock_service(mach_host_self(), cid, &cclock);
         clock_get_time(cclock, &mts);
         mach_port_deallocate(mach_task_self(), cclock);
         spec->tv_sec = mts.tv_sec;
         spec->tv_nsec = mts.tv_nsec;
-    } else if (source == JANET_TIME_MONOTONIC) {
-        clock_serv_t cclock;
-        int nsecs;
-        mach_msg_type_number_t count;
-        host_get_clock_service(mach_host_self(), SYSTEM_CLOCK, &cclock);
-        clock_get_attributes(cclock, CLOCK_GET_TIME_RES, (clock_attr_t)&nsecs, &count);
-        mach_port_deallocate(mach_task_self(), cclock);
-        clock_getres(CLOCK_MONOTONIC, spec);
-    }
-    if (source == JANET_TIME_CPUTIME) {
-        clock_t tmp = clock();
-        spec->tv_sec = tmp / CLOCKS_PER_SEC;
-        spec->tv_nsec = ((tmp - (spec->tv_sec * CLOCKS_PER_SEC)) * 1000000000) / CLOCKS_PER_SEC;
     }
     return 0;
 }