[Morello] Implement LDUR/STUR fallback for LDR/STR in altbase mode

[Morello] Implement LDUR/STUR fallback for LDR/STR in altbase mode

Give the ops saner names instead of _2, _3, etc.

gas/ChangeLog:

2020-10-20  Siddhesh Poyarekar  <siddesh.poyarekar@arm.com>

	* config/tc-aarch64.c (parse_operand): Reuse ADDR_UIMM12 code
	in CAPADDR_UIMM9.
	(try_to_encode_as_unscaled_ldst, fix_insn): Add support for
	altbase loads and stores.
	* testsuite/gas/aarch64/morello_ldst-c64.d: Add tests.
	* testsuite/gas/aarch64/morello_ldst.d: Likewise.
	* testsuite/gas/aarch64/morello_ldst.s: Likewise.

include/ChangeLog:

2020-10-20  Siddhesh Poyarekar  <siddesh.poyarekar@arm.com>

	* opcode/aarch64.h (aarch64_op): Add members for altbase LDUR
	and STUR.  Rename existing LDUR ops.

opcodes/ChangeLog:

2020-10-20  Siddhesh Poyarekar  <siddesh.poyarekar@arm.com>

	* aarch64-tbl.h (aarch64_opcode_table): Change OP of LDUR/STUR
	instructions.
	* aarch64-opc-2.c: Regenerate.