linux: export AT_HWCAP and AT_HWCAP2 on aarch64

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.

Reviewed By: mhorne, emaste
Sponsored By: EPSRC
Differential Revision: https://reviews.freebsd.org/D30540

(cherry picked from commit b501b2ae52f3ccb384584dcbb5dc4a4568cc0096)