On AArch64, the performance counter registers usually are
restricted and not accessible from user space.

On macOS, we currently use mach_absolute_time() as timer on
aarch64. This measures wallclock time but with a very coarse
resolution.

There is a private API, kperf, that one can use for getting
high precision timers though. Unfortunately, it requires running
the checkasm binary as root (e.g. with sudo).

Also, as it is a private, undocumented API, it can potentially
change at any time.

This is handled by adding a new meson build option, for switching
to this timer. If the timer source in checkasm could be changed
at runtime with an option, this wouldn't need to be a build time
option.

This allows getting benchmarks like this:

mc_8tap_regular_w16_hv_8bpc_c:              1522.1 ( 1.00x)
mc_8tap_regular_w16_hv_8bpc_neon:            331.8 ( 4.59x)

Instead of this:

mc_8tap_regular_w16_hv_8bpc_c:                 9.0 ( 1.00x)
mc_8tap...