Remake support for SMP kernel on UP cpu:

• Use new option SMP_ON_UP instead of (mis)using specific CPU type. By this, any SMP kernel can be compiled with SMP_ON_UP support.
• Enable runtime detection of CPU multiprocessor extensions only if SMP_ON_UP option is used. In other cases (pure SMP or UP), statically compile only required variant.
• Don't leak multiprocessor instructions to UP kernel.
• Correctly handle data cache write back to point of unification. DCCMVAU is supported on all armv7 cpus.
• For SMP_ON_UP kernels, detect proper TTB flags on runtime.

Differential Revision: https://reviews.freebsd.org/D9133