internal/cpu: implement CPU capability detection for openbsd/arm64

[4a6f656c]

Most arm64 runtimes make use of auxv HWCAP data in order to determine what CPU features are available. OpenBSD does not currently provide auxv - we either need a way for the kernel to provide this information, or we need to implement CPU capability detection in userland.

[gopherbot]

Change https://golang.org/cl/174129 mentions this issue: runtime: initialise cpu.HWCap on openbsd/arm64

[tklauser]

We'll probably face the same problem for netbsd/arm64 (#30824) and freebsd/arm64 (#24715), I assume?

[valpackett]

on freebsd/arm64 it's done here: https://github.com/MikaelUrankar/go/commit/b46aa4ee007eedd2b7be40ac46dce507a39225b3#diff-ea01e2d7a6eb362c2114bdfca93fdaf2R42

(that code should also work on modern linux btw)

[tklauser]

AFICT that code requires emulation of the MRS instruction in the kernel (as described in https://github.com/torvalds/linux/blob/master/Documentation/arm64/cpu-feature-registers.txt for Linux). I'm not sure if OpenBSD offers this.

[gopherbot]

Change https://golang.org/cl/263337 mentions this issue: cpu: avoid illegal instruction on openbsd/arm64