Make elf_aux_info() as public libc function.

• Teach elf aux vector functions about newly added AT_HWCAP and AT_HWCAP2 vectors.
• Export _elf_aux_info() as new public libc function elf_aux_info(3)

The elf_aux_info(3) should be considered as FreeBSD counterpart of glibc
getauxval() with more robust interface.

Note:
We cannot name this new function as getauxval(), with glibc compatible
interface. Some ports autodetect its existence and then expects that all
Linux specific AT_<*> vectors are defined and implemented.

MFC after: 1 month
Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D12743