RISC-V: Adding T-Head MemPair extension

This patch adds support for the T-Head MemPair instructions.
The patch uses the T-Head specific decoder and translation.

Co-developed-by: LIU Zhiwei <zhiwei_liu@linux.alibaba.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Christoph Müllner <christoph.muellner@vrull.eu>
Message-Id: <20230131202013.2541053-9-christoph.muellner@vrull.eu>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>

target/riscv/cpu.h

@@ -479,6 +479,7 @@ struct RISCVCPUConfig {
     bool ext_xtheadcmo;
     bool ext_xtheadcondmov;
     bool ext_xtheadmac;
+    bool ext_xtheadmempair;
     bool ext_xtheadsync;
     bool ext_XVentanaCondOps;