Add list of valid CPUTYPE flags for arm/arm64 to make.conf example

Summary:
Adds a list of valid CPUTYPE flags for arm and arm64 architectures
List taken from share/mk/bsd.cpu.mk

Submitted by: Daniel Engberg
Reviewed By: imp
Differential Revision: https://reviews.freebsd.org/D20315

share/examples/etc/make.conf

@@ -49,6 +49,12 @@
 #			icelake-client, cannonlake, knm, skylake-avx512, knl,
 #			goldmont, skylake, broadwell, haswell, ivybridge,
 #			sandybridge, westmere, nehalem, silvermont, bonnell
+#   ARM architecture:	armv5, armv5te, armv6, armv6t2, arm1176jzf-s, armv7,
+#			armv7-a, armv7ve, generic-armv7-a, cortex-a5,
+#			cortex-a7, cortex-a8, cortex-a9, cortex-a12,
+#			cortex-a15, cortex-a17
+#   ARM64 architechture:	cortex-a53, cortex-a57, cortex-a72,
+#				exynos-m1
 #
 # (?= allows to buildworld for a different CPUTYPE.)
 #