enable CNTVCT_EL0 by default on aarch64

[stevengj]

As suggested here.

Should work on MacOS and Linux at least? Pro: most ARM users will get a cycle counter that way, and they might not know to enable it otherwise (we silently revert to ESTIMATE plans. Con: it's possible that on some systems this register won't be available … but at least it will fail loudly (crash) in that case, and this doesn't seem to be a problem for the most common ARM platforms these days(?)

cc @rdolbeau