Read System Clock for Monotonic Time

[nlordell]

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 and 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

[nlordell]

Disclaimer: I tested the new janet_gettime code on macOS 15 (so not on the system that it was intended for), and it seems to produce decent results.

Normal implementation with clock_gettime:

repl:1:> (os/clock :realtime)
1755506717.49195
repl:2:> (os/clock :monotonic)
65016.336813
repl:3:> (os/clock :cputime)
0.013324

Implementation with the host_get_clock_service-based implementation used by OS X <10.12:

repl:1:> (os/clock :realtime)
1755506806.85266
repl:2:> (os/clock :monotonic)
31583.262315791
repl:3:> (os/clock :cputime)
0.01402

Ideally, this would be tested on an OS X system <10.12 (but I don't have access to one).