linux: export AT_HWCAP and AT_HWCAP2 on aarch64

[trasz]

Summary

The flag values seem to be the same between Linux and FreeBSD.
Comparing to a Linux VM on the same hardware, we're missing
HWCAP_EVTSTRM, HWCAP_CPUID, HWCAP_DCPOP, HWCAP_USCAT, HWCAP_PACA,
and HWCAP_PACG.

[trasz]

(Tinderboxed.)

[emaste]

It's intentional that the same bits are used, but IMO we should have some way to help ensure that remains so, even if just a more explicit comment in sys/arm64/include/elf.h.

[trasz]

It already suggests that's the case, though: "XXX: The following bits (from CPUID to FLAGM) were originally incorrect, but later changed to match the Linux definitions."

[emaste]

Indeed, hence "a more explicit comment" - it's only after this change that they must match.