RISC-V: Add reductions instructions for T-Head VECTOR vendor extension

RISC-V: Add reductions 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 reductions 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
	reductions instructions.
	* testsuite/gas/riscv/x-thead-vector.s: Likewise.

opcodes/ChangeLog:

	* riscv-opc.c: Likewise.