conf: Document why we have ARM64 and RISCV options

These are needed for the 'cpu ARM64' and 'cpu RISCV' options in these
architecture's config files. cpu lines are non-optional in config(8), so
we must define them here. There's no other use for them in the tree.

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sys/conf/options.arm64

@@ -1,6 +1,6 @@
 # $FreeBSD$
 
-ARM64				opt_global.h
+ARM64				opt_global.h		# for cpu ARM64 to work
 INTRNG				opt_global.h
 SOCDEV_PA			opt_global.h
 THUNDERX_PASS_1_1_ERRATA	opt_global.h

sys/conf/options.riscv

@@ -1,5 +1,5 @@
 # $FreeBSD$
 
-RISCV				opt_global.h
+RISCV				opt_global.h	# For cpu RISCV to work
 FPE				opt_global.h
 INTRNG				opt_global.h