[BINUTILS, AArch64, 1/2] Add new LDGM/STGM instruction

[BINUTILS, AArch64, 1/2] Add new LDGM/STGM instruction

This patch adds the new LDGM/STGM instructions of the
Armv8.5-A Memory Tagging Extension. This is part of the changes
that have been introduced recently in the 00bet10 release

The instructions are as follows:
LDGM Xt, [<Xn|SP>]
STGM Xt, [<Xn|SP>]

*** gas/ChangeLog ***

2019-04-16  Sudakshina Das  <sudi.das@arm.com>

	Backported from mainline
	2019-04-11  Sudakshina Das  <sudi.das@arm.com>

	* testsuite/gas/aarch64/armv8_5-a-memtag.d: New tests for ldgm and stgm.
	* 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 ***

2019-04-16  Sudakshina Das  <sudi.das@arm.com>

	Backported from mainline
	2019-04-11  Sudakshina Das  <sudi.das@arm.com>

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

(cherry picked from commit e54010f1aeb050cb9d65862a0afe9095a7a85f27)