arm64 equivalents of PR_SET_TSC/ARCH_SET_CPUID

[Kyle Huey]

There is ongoing work by Yichao Yu to make rr, a userspace record and
replay debugger[0], production quality on arm64[1]. One of the bigger
remaining issues is the kernel's emulation of accesses to certain
system registers[2] that reflect timing and CPU capabilities and are
either non-deterministic or can vary from processor to processor. We
would like to add the ability to tell the kernel to decline to emulate
these instructions for a given task and pass that responsibility onto
the supervising rr ptracer. There are analogous processor features and
disabling mechanisms on x86. The RDTSC instruction is controlled by
prctl(PR_SET_TSC) and the CPUID instruction is controlled (when the
hardware allows) by arch_prctl(ARCH_SET_CPUID).

The questions I'd like to raise are:

1. Is it appropriate to reuse PR_SET_TSC for roughly equivalent
functionality on AArch64? (even if the AArch64 feature is not actually
named Time Stamp Counter).
2. Likewise for ARCH_SET_CPUID
3. Since arch_prctl is x86-only, does it make more sense to add
arch_prctl to arm64 or to duplicate ARCH_SET_CPUID into the prctl
world? (e.g. a PR_SET_CPUID that works on both x86/arm64)

- Kyle

[0] https://rr-project.org/
[1] https://github.com/rr-debugger/rr/issues/3234
[2] e.g. CNTVCT_EL0 and MIDR_EL1, among others