[AArch64] Add a "compnum" feature

[AArch64] Add a "compnum" feature

This patch adds a named "compnum" feature for the ARMv8.3-A FCADD
and FCMLA extensions.

include/
	* opcode/aarch64.h (AARCH64_FEATURE_COMPNUM): New macro.
	(AARCH64_ARCH_V8_3): Include AARCH64_FEATURE_COMPNUM.

opcodes/
	* aarch64-tbl.h (aarch64_feature_simd_v8_3): Replace with...
	(aarch64_feature_compnum): ...this.
	(SIMD_V8_3): Replace with...
	(COMPNUM): ...this.
	(CNUM_INSN): New macro.
	(aarch64_opcode_table): Use it for the complex number instructions.

gas/
	* doc/c-aarch64.texi: Add a "compnum" entry.
	* config/tc-aarch64.c (aarch64_features): Likewise,
	* testsuite/gas/aarch64/advsimd-compnum.s: New test.
	* testsuite/gas/aarch64/advsimd-compnum.d: Likewise.