[AArch64] Add support for GMID_EL1 register for +memtag

[AArch64] Add support for GMID_EL1 register for +memtag

We're missing support for the GMID_EL1 system register from the Memory Tagging Extension in binutils.
This is specified at:
https://developer.arm.com/docs/ddi0595/latest/aarch64-system-registers/gmid_el1

This simple patch adds the support for this read-only register.
Tested make check on gas.

	Backport from mainline
	2019-07-23  Kyrylo Tkachov  <kyrylo.tkachov@arm.com>

	* aarch64-opc.c (aarch64_sys_regs): Add gmid_el1 entry.
	(aarch64_sys_reg_supported_p): Handle gmid_el1 encoding.

	Backport from mainline
	2019-07-23  Kyrylo Tkachov  <kyrylo.tkachov@arm.com>

	* testsuite/gas/aarch64/sysreg-4.s: Test gmid_el1 read.
	* testsuite/gas/aarch64/sysreg-4.d: Update expected output.
	* testsuite/gas/aarch64/illegal-sysreg-4.l: Likewise.