target/riscv: Use target_ulong for the DisasContext misa

The is_32bit() check in translate.c expects a 64-bit guest to have a
64-bit misa value otherwise the macro check won't work. This patches
fixes that and fixes a Coverity issue at the same time.

Fixes: CID 1453107
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Message-id: c00176c7518c2a7b4de3eec320b6a683ab56f705.1622435221.git.alistair.francis@wdc.com

target/riscv/translate.c

@@ -47,7 +47,7 @@ typedef struct DisasContext {
     bool virt_enabled;
     uint32_t opcode;
     uint32_t mstatus_fs;
-    uint32_t misa;
+    target_ulong misa;
     uint32_t mem_idx;
     /* Remember the rounding mode encoded in the previous fp instruction,
        which we have already installed into env->fp_status.  Or -1 for