MFC 323580,323933,323934,324814,324817: Enable AT_HWCAP on arm.

I reused the SV_HWCAP stub to cover the sv_hwcap2 field as well.

323580:
Add AT_HWCAP flags for VFP settings for FreeBSD/arm.

These flags match the meaning and value of flags in Linux, though
Linux has many more flags.

323933:
Correct HWCAP_VFP3* values to match Linux.

323934:
Detect NEON and set HWCAP_NEON if present.

324814:
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.

324817:
Fullify implementation of AT_HWCAP and AT_HWCAP2 for ARMv6,7.
This makes elf_aux_info(3) useable for ARM ports.

Tested by: mmel