[AArch64] Add separate feature flag for weaker release consistent load insns

[AArch64] Add separate feature flag for weaker release consistent load insns

The weaker release consistency support of ARMv8.3-A is allowed as an optional
extension for ARMv8.2-A, so separate command line option and feature flag is
added: -march=armv8.2-a+rcpc turns LDAPR, LDAPRB, LDAPRH instructions on.

opcodes/
	* aarch64-tbl.h (RCPC, RCPC_INSN): Define.
	(aarch64_opcode_table): Use RCPC_INSN.

include/
	* opcode/aarch64.h (AARCH64_FEATURE_RCPC): Define.
	(AARCH64_ARCH_V8_3): Update.

gas/
	* config/tc-aarch64.c (aarch64_features): Add rcpc.
	* doc/c-aarch64.texi (AArch64 Extensions): Document rcpc.
	* testsuite/gas/aarch64/ldst-exclusive-armv8_3.d: Rename to ...
	* testsuite/gas/aarch64/ldst-rcpc.d: This.
	* testsuite/gas/aarch64/ldst-exclusive-armv8_3.s: Rename to ...
	* testsuite/gas/aarch64/ldst-rcpc.s: This.
	* testsuite/gas/aarch64/ldst-rcpc-armv8_2.d: New test.