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

sys/conf/options.powerpc

@@ -15,6 +15,8 @@ FPU_EMU
 
 COMPAT_FREEBSD32	opt_global.h
 
+ISA_206_ATOMICS		opt_global.h
+
 MOEA64_STATS		opt_pmap.h
 AMIGAONE		opt_platform.h
 MIKROTIK		opt_platform.h