Using the CPU feature detection framework, add a CPU_FEAT_EARLY_BOOT
check for the Memory Tagging Extension, and a HWCAP for FEAT_MTE.
Like Pointer Authentication (sys/arm64/arm64/ptrauth.c), MTE logic will
be kept within it's own file. In order to use the MTE registers and
instructions, this file needs the "-march=armv8.5+memtag" compiler flag.
Sponsored by: Arm Ltd
Signed-off-by: Harry Moulton <harry.moulton@arm.com>
Co-authored-by: Andrew Turner <andrew@FreeBSD.org>