aarch64: Add +pauth to -march

aarch64: Add +pauth to -march

New +pauth (Pointer Authentication from Armv8.3-A) feature option for
-march command line option.

Please note that majority of PAUTH instructions are implemented behind HINT
instruction. PAUTH stays an Armv8.3-A feature but now can be assigned to other
architectures or CPUs.

gcc/ChangeLog:

	* config/aarch64/aarch64-option-extensions.def
	(AARCH64_OPT_EXTENSION): New +pauth option in -march for AArch64.
	* config/aarch64/aarch64.h (AARCH64_FL_PAUTH): New pauth extension bitmask.
	(AARCH64_ISA_PUATH): New ISA bitmask for PAUTH.
	(AARCH64_FL_FOR_ARCH8_3): Add PAUTH to Armv8.3-A.
	(TARGET_PAUTH): New target mask to isolate PAUTH instructions.
	* config/aarch64/aarch64.md (do_return): Condition set to TARGET_PAUTH.
	* doc/invoke.texi: Update docs for +flagm and +pauth.