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

include/ChangeLog:

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

opcodes/ChangeLog:

	* riscv-opc.c: Likewise.

include/opcode/riscv-opc.h

@@ -2897,6 +2897,18 @@
 #define MASK_TH_VSBCVXM 0xfe00707f
 #define MATCH_TH_VWMACCSUVV 0xf8002057
 #define MASK_TH_VWMACCSUVV 0xfc00707f
+#define MATCH_TH_VAADDVV 0x90000057
+#define MASK_TH_VAADDVV 0xfc00707f
+#define MATCH_TH_VAADDVX 0x90004057
+#define MASK_TH_VAADDVX 0xfc00707f
+#define MATCH_TH_VAADDVI 0x90003057
+#define MASK_TH_VAADDVI 0xfc00707f
+#define MATCH_TH_VASUBVV 0x98000057
+#define MASK_TH_VASUBVV 0xfc00707f
+#define MATCH_TH_VASUBVX 0x98004057
+#define MASK_TH_VASUBVX 0xfc00707f
+#define MATCH_TH_VWSMACCSUVV 0xf8000057
+#define MASK_TH_VWSMACCSUVV 0xfc00707f
 /* Vendor-specific (Ventana Microsystems) XVentanaCondOps instructions */
 #define MATCH_VT_MASKC 0x607b
 #define MASK_VT_MASKC 0xfe00707f