[Morello] Allow lo12 relocations for alternate base ld/st

[Morello] Allow lo12 relocations for alternate base ld/st

This brings feature parity between the regular, Morello and alternate
base load/store instructions.

bfd/ChangeLog:

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

	* elfxx-aarch64.c (reencode_ldst_pos_imm): Support loads and
	stores with alternate base.

gas/ChangeLog:

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

	* config/tc-aarch64.c (ldst_lo12_determine_real_reloc_type):
	Support alternate base loads and stores.
	(parse_operands): Support relocations for alternate base
	address operands.
	* testsuite/gas/aarch64/morello-ldst-reloc.d: New file.
	* testsuite/gas/aarch64/morello-ldst-reloc.s: New file.