Allow an SMP kernel to boot on Cortex-A8:

• Rename ARM_HAVE_MP_EXTENSIONS to ARM_USE_MP_EXTENSIONS and extend it to handle more cases, including when SMP is not enabled.
• Check ARM_USE_MP_EXTENSIONS when building for ARMv7+, even if no SMP.
• Use ARM_USE_MP_EXTENSIONS in pmap-v6.c to detect when to set PRRR_NS1.

With this we should be able to boot on all ARMv7+ Cortex-A cores with
32-bit support.

Reviewed by: mmel, imp (earlier version)
Relnotes: yes
Sponsored by: ABT Systems Ltd
Differential Revision: https://reviews.freebsd.org/D8335