aarch64 timer is available to userspace via arch register.
clang's __builtin_readcyclecounter is wrong for aarch64 (reads the PMU
cycle counter instead of the archi-timer register), so we don't use it.

my_rdtsc unit-test on AWS m6g shows:
frequency: 121830845
resolution: 1
overhead: 1

This counter is not strictly increasing, but it is non-decreasing.