aarch64: Add id_aa64isar2_el1

aarch64: Add id_aa64isar2_el1

Armv8.8-A defines a read-only system register called id_aa64isar2_el1.
The register was previously RES0 and should therefore be accepted
at all architecture levels.
[https://developer.arm.com/documentation/ddi0595/2021-09/AArch64-Registers/ID-AA64ISAR2-EL1--AArch64-Instruction-Set-Attribute-Register-2?lang=en]

opcodes/
	* aarch64-opc.c (aarch64_sys_regs): Add id_aa64isar2_el1.

gas/
	* testsuite/gas/aarch64/sysreg-diagnostic.s: Test writes to
	id_aa64isar2_el1.
	* testsuite/gas/aarch64/sysreg-diagnostic.d: Update accordingly.
	* testsuite/gas/aarch64/sysreg-diagnostic.l: Likewise.
	* testsuite/gas/aarch64/sysreg.s: Test reads from
	id_aa64isar2_el1.
	* testsuite/gas/aarch64/sysreg.d: Update accordingly.