Add AT_HWCAP2 ELF auxiliary vector.

• allocate value for new AT_HWCAP2 auxiliary vector on all platforms.
• expand 'struct sysentvec' by new 'u_long *sv_hwcap2', in exactly same way as for AT_HWCAP.

MFC after: 1 month
Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D12699