RISC-V: Add CSRs for T-Head VECTOR vendor extension

RISC-V: Add CSRs for T-Head VECTOR vendor extension

T-Head has a range of vendor-specific instructions.
Therefore it makes sense to group them into smaller chunks
in form of vendor extensions.

This patch adds the CSRs for XTheadVector. Because of the
conflict between encoding and teh 'V' extension, it is implemented
by alias. The 'th' prefix and the "XTheadVector" extension are
documented in a PR for the RISC-V toolchain conventions ([1]).

[1] https://github.com/riscv-non-isa/riscv-toolchain-conventions/pull/19

Co-developed-by: Lifang Xia <lifang_xia@linux.alibaba.com>
Co-developed-by: Christoph Müllner <christoph.muellner@vrull.eu>
gas/ChangeLog:

	* config/tc-riscv.c (enum riscv_csr_class): Add the class for
	the CSRs of the "XTheadVector" extension.
	(riscv_csr_address): Likewise.
	* testsuite/gas/riscv/x-thead-vector-csr-warn.d: New test.
	* testsuite/gas/riscv/x-thead-vector-csr-warn.l: New test.
	* testsuite/gas/riscv/x-thead-vector-csr.d: New test.
	* testsuite/gas/riscv/x-thead-vector-csr.s: New test.

include/ChangeLog:

	* opcode/riscv-opc.h (DECLARE_CSR_ALIAS): Likewise.

opcodes/ChangeLog:

	* riscv-dis.c (print_insn_args): Prefix the CSRs disassembly with 'th'.