Make ISA_206_ATOMICS a kernel option

Summary:
To make it easier to build a kernel with PowerISA 2.06 atomics (sub-word
atomics), add a kernel config option. User space still needs to specify
it as a CFLAG but that seems easier to do than for the kernel config.

Reviewed By: luporl
Differential Revision: https://reviews.freebsd.org/D29809