Allow running `checkasm` in user mode on AARCH64.

The performance counter you use in checkasm, namely pmccntr_el0 is only available in kernel mode applications. So, unless you are root, you cannot run checkasm on AARCH64 machines. This change fixes this problem.

The cntvct_el0 now used is a virtualized version of essentially the same counter. See also here: https://developer.arm.com/documentation/ddi0595/2020-12/AArch64-Registers/CNTVCT-EL0--Counter-timer-Virtual-Count-register

For good measure, I also threw in a isb (instruction barrier).