aarch64: Add +flagm to -march

aarch64: Add +flagm to -march

New +flagm (Condition flag manipulation) feature option for -march command line
option.

Please note that FLAGM stays a Armv8.4-A feature but now can be
assigned to other architectures or CPUs.

gcc/ChangeLog:

	* config/aarch64/aarch64-option-extensions.def
	(AARCH64_OPT_EXTENSION): New +flagm option in -march for AArch64.
	* config/aarch64/aarch64.h (AARCH64_FL_FLAGM): Add new flagm extension bit
	mask.
	(AARCH64_FL_FOR_ARCH8_4): Add flagm to Armv8.4-A.
	* doc/invoke.texi: Update docs with +flagm.