arm64: enable options NUMA in GENERIC

As with amd64 NUMA is required for reasonable operation on big-iron
arm64 systems and is expected to have no significant impact on small
systems.  Enable it now for wider testing in advance of FreeBSD 13.0.

You can use the 'vm.ndomains' sysctl to see if multiple domains are in
use - for example (from Cavium/Marvell ThunderX2):

# sysctl vm.ndomains
vm.ndomains: 2

No objection:	manu
Sponsored by:	The FreeBSD Foundation

sys/arm64/conf/GENERIC

@@ -25,6 +25,7 @@ makeoptions	DEBUG=-g		# Build kernel with gdb(1) debug symbols
 makeoptions	WITH_CTF=1		# Run ctfconvert(1) for DTrace support
 
 options 	SCHED_ULE		# ULE scheduler
+options 	NUMA			# Non-Uniform Memory Architecture support
 options 	PREEMPTION		# Enable kernel thread preemption
 options 	VIMAGE			# Subsystem virtualization, e.g. VNET
 options 	INET			# InterNETworking