The default timer register pmccntr_el0 usually requires enabling
access with e.g. a kernel module (while it is accessible by
default on Windows). On Linux, the default for checkasm benchmarks
is to use perf (if suitable headers are available) though.

On macOS, using cntvct_el0 gives measurements with the same
magnitude as mach_absolute_time (which is used currently), but
possibly with a little less overhead/noise.
Signed-off-by: Martin Storsjö <martin@martin.st>