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

RISC-V: Add load/store segment 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 provides load/store segment instructions for T-Head VECTOR
vendor extension, which same as the "Zvlsseg" extension in RVI 0.71 vector
extension, but belongs to 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 test.
	* testsuite/gas/riscv/x-thead-vector.s: Likewise.

include/ChangeLog:

	* opcode/riscv-opc.h (MATCH_TH_VLSEG2BV): New.

opcodes/ChangeLog:

	* riscv-opc.c: Likewise.