RISC-V: Add load/store instructions for T-Head VECTOR vendor extension

RISC-V: Add load/store instructions 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 load/store instructions for the "XTheadVector"
extension. 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:

	* testsuite/gas/riscv/x-thead-vector.d: Add tests for
	load/store instructions.
	* testsuite/gas/riscv/x-thead-vector.s: Likewise.

include/ChangeLog:

	* opcode/riscv-opc.h (MATCH_TH_VLBV): New.

opcodes/ChangeLog:

	* riscv-opc.c: Likewise.