aarch64: Remove +mpamv2 and support for MPAMV2_VID.

There are no known implementations of the MPAMV2_VID extension, and
Arm has decided to remove it from the AArch64 architecture specification.
This patch implements that withdrawal.  The only purpose of the +mpamv2
architectural feature extension was to enable the instructions added
by MPAMV2_VID in the assembler and these have also been removed.  The
feature extension is thus redundant and has been removed.  The sysreg
specifications have been updated to conform with similar sysregs that
have no associated instructions in the feature.

gcc/ChangeLog:

	* config/aarch64/aarch64-option-extensions.def (AARCH64_OPT_EXTENSION):
	Remove +mpamv2.
	* config/aarch64/aarch64-sys-regs.def: Remove sysregs associated
	with MPAMv2_VID and adjust other related sysregs to use the
	base permitted architecture.
	* doc/invoke.texi (aarch64: +mpamv2): Remove option.