checkasm: Add support for the private macOS kperf API for benchmarking

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...