Hi all,

this adds targeting-checking entities for PACBTI in testsuite
framework.

Pre-approved with the requested changes here
<https://gcc.gnu.org/pipermail/gcc-patches/2021-December/586331.html>.

gcc/testsuite/ChangeLog:

	* testsuite/lib/target-supports.exp:
	(check_effective_target_arm_pacbti_hw): New.
	* doc/sourcebuild.texi: Document arm_pacbti_hw.

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