don't use CLOCK_SGI_CYCLE on Linux ARM systems

[stevengj]

We are seeing a build error on Linux with musl libc on ARM (JuliaPackaging/Yggdrasil#13550 (comment)) with --enable-armv8-cntvct-el0 because CLOCK_SGI_CYCLE is defined (for some reason) by the musl time.h header, so then cycle.h ends up trying to define ticks twice.

Probably at least two changes:

1. In cycle.h, move all of the implementations based on high-level functions like clock_gettime to go below the low-level cycle counters, so that the cycle counters are preferred.
2. Don't use CLOCK_SGI_CYCLE on non-SGI systems (i.e. on no modern systems)?

[matteo-frigo]

You are in the best position to fix this until it works on both musl and apple and normal linux.