linux(4): Add elf_hwcap2 to x86
On x86 Linux via AT_HWCAP2 the user controlled (by tunables) processor
capabilities are exposed.
Reviewed by:
Differential Revision: https://reviews.freebsd.org/D41165
MFC after: 2 weeks
(cherry picked from commit 4281dab8bc11b3b2f3b58be5e0f67806329bddae)