This patch adds the -march feature +pacbti to Armv8.1-M Mainline.
This feature enables pointer signing and authentication instructions
on M-class architectures.

Pre-approved here
<https://gcc.gnu.org/pipermail/gcc-patches/2021-December/586144.html>.

gcc/Changelog:

	* config/arm/arm.h (TARGET_HAVE_PACBTI): New macro.
	* config/arm/arm-cpus.in (pacbti): New feature.
	* doc/invoke.texi (Arm Options): Document it.

Co-Authored-By: Tejas Belagod  <tbelagod@arm.com>