AArch64: Add new STZGM instruction for Armv8.5-A Memory Tagging Extension.

This patch is part of a series of patches to introduce a few changes to the
Armv8.5-A Memory Tagging Extension. This patch adds the new STZGM instruction.

STGZM Xt, [<Xn|SP>]

Committed on behalf of Sudakshina Das.

*** gas/ChangeLog ***

	* testsuite/gas/aarch64/armv8_5-a-memtag.d: New tests for stzgm.
	* testsuite/gas/aarch64/armv8_5-a-memtag.s: Likewise.
	* testsuite/gas/aarch64/illegal-memtag.l: Likewise.
	* testsuite/gas/aarch64/illegal-memtag.s: Likewise.

*** opcodes/ChangeLog ***

	* aarch64-asm-2.c: Regenerated.
	* aarch64-dis-2.c: Likewise.
	* aarch64-opc-2.c: Likewise.
	* aarch64-tbl.h (aarch64_opcode): Add new stzgm.

(cherry picked from commit 20a4ca55)