aarch64: Add support for Common Short Sequence Compression extension

This patch adds support for the CSSC extension and its corresponding
instructions: ABS, CNT, CTZ, SMAX, UMAX, SMIN, UMIN.

gas/ChangeLog:

        * config/tc-aarch64.c (parse_operands): Handle new operand types.
        * doc/c-aarch64.texi: Document new extension.
        * testsuite/gas/aarch64/cssc.d: New test.
        * testsuite/gas/aarch64/cssc.s: New test.

include/ChangeLog:

        * opcode/aarch64.h (AARCH64_FEATURE_CSSC): New feature Macro.
        (enum aarch64_opnd): New operand types.
        (enum aarch64_insn_class): New instruction class.

opcodes/ChangeLog:

	* aarch64-asm-2.c: Regenerate.
	* aarch64-dis-2.c: Regenerate.
	* aarch64-opc-2.c: Regenerate.
	* aarch64-opc.c (operand_general_constraint_met_p): Update for new
	operand types.
	(aarch64_print_operand): Likewise.
	* aarch64-opc.h (enum aarch64_field_kind): Declare FLD_CSSC_imm8 field.
	* aarch64-tbl.h (aarch64_feature_cssc): Define new feature set.
	(CSSC): Define new feature set Macro.
	(CSSC_INSN): Define new instruction type.
	(aarch64_opcode_table): Add new instructions.