RISC-V: Add Zcmt instructions and csr.

This patch supports Zcmt[1] instruction 'cm.jt' and 'cm.jalt'.
Add new CSR jvt for tablejump using. Since 'cm.jt' and 'cm.jalt'
have the same instructiong encoding, use 'match_cm_jt' and 'match_cm_jalt'
check the 'zcmt_index' field to distinguish them.

[1] https://github.com/riscvarchive/riscv-code-size-reduction/releases

Co-Authored by: Charlie Keaney <charlie.keaney@embecosm.com>
Co-Authored by: Mary Bennett <mary.bennett@embecosm.com>
Co-Authored by: Nandni Jamnadas <nandni.jamnadas@embecosm.com>
Co-Authored by: Sinan Lin <sinan.lin@linux.alibaba.com>
Co-Authored by: Simon Cook <simon.cook@embecosm.com>
Co-Authored by: Shihua Liao <shihua@iscas.ac.cn>
Co-Authored by: Yulong Shi <yulong@iscas.ac.cn>

bfd/ChangeLog:

	* elfxx-riscv.c (riscv_multi_subset_supports): New extension.
	(riscv_multi_subset_supports_ext): Ditto.

gas/ChangeLog:

	* config/tc-riscv.c (enum riscv_csr_class): New CSR.
	(riscv_csr_address): Ditto.
	(validate_riscv_insn): New operand.
	(riscv_ip): Ditto.
	* testsuite/gas/riscv/csr-version-1p10.d: New CSR.
	* testsuite/gas/riscv/csr-version-1p10.l: Ditto.
	* testsuite/gas/riscv/csr-version-1p11.d: Ditto.
	* testsuite/gas/riscv/csr-version-1p11.l: Ditto.
	* testsuite/gas/riscv/csr-version-1p12.d: Ditto.
	* testsuite/gas/riscv/csr-version-1p12.l: Ditto.
	* testsuite/gas/riscv/csr.s: Ditto.
	* testsuite/gas/riscv/march-help.l: New extension.
	* testsuite/gas/riscv/zcmt-fail.d: New test.
	* testsuite/gas/riscv/zcmt-fail.l: New test.
	* testsuite/gas/riscv/zcmt-fail.s: New test.
	* testsuite/gas/riscv/zcmt.d: New test.
	* testsuite/gas/riscv/zcmt.s: New test.

include/ChangeLog:

	* opcode/riscv-opc.h (MATCH_CM_JT): New opcode.
	(MASK_CM_JT): New mask.
	(MATCH_CM_JALT): New opcode.
	(MASK_CM_JALT): New mask.
	(CSR_JVT): New CSR.
	(DECLARE_INSN): New declaration.
	(DECLARE_CSR): Ditto.
	* opcode/riscv.h (EXTRACT_ZCMT_INDEX): New marco.
	(ENCODE_ZCMT_INDEX): Ditto.
	(enum riscv_insn_class): New class.

opcodes/ChangeLog:

	* riscv-dis.c (print_insn_args): New operand.
	* riscv-opc.c (match_cm_jt): New function.
	(match_cm_jalt): Ditto.
19 files changed