procstat: Recognize HWCAP and HWCAP2 with auxv command

The two most recent additions to the elf auxiliary vector are
HWCAP and HWCAP2 which describe platform specific cpu capabilities.

Make procstat recognize these fields so that they aren't displayed
as UNKNOWN.

Reviewed by: trociny, markj
Approved by: markj (mentor)
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D20582