Add AT_HWCAP / AT_HWCAP2 to elf64_sysvec_v2.

Summary:
I was working on implementing ifuncs on powerpc64 elfv2 today, and I suddenly
realized that the reason I was having so much trouble with AT_HWCAP and
AT_HWCAP2 is they are missing from the sysentvec.

After adding them, the auxv is being filled like it should.

Submitted by: Brandon Bergren (git_bdragon.rtk0.net)
Differential Revision: https://reviews.freebsd.org/D18575