runtime: use mach_absolute_time for runtime.nanotime
The existing darwin/amd64 implementation of runtime.nanotime returns the
wallclock time, which results in timers not functioning properly when
system time runs backwards. By implementing the algorithm used by the
darwin syscall mach_absolute_time, timers will function as expected.
The algorithm is described at
https://opensource.apple.com/source/xnu/xnu-3248.60.10/libsyscall/wrappers/mach_absolute_time.s