target/riscv: store RISCVCPUDef struct directly in the class

Prepare for adding more fields to RISCVCPUDef and reading them in
riscv_cpu_init: instead of storing the misa_mxl_max field in
RISCVCPUClass, ensure that there's always a valid RISCVCPUDef struct
and go through it.

Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

hw/riscv/boot.c

@@ -37,7 +37,7 @@
 bool riscv_is_32bit(RISCVHartArrayState *harts)
 {
     RISCVCPUClass *mcc = RISCV_CPU_GET_CLASS(&harts->harts[0]);
-    return mcc->misa_mxl_max == MXL_RV32;
+    return mcc->def->misa_mxl_max == MXL_RV32;
 }
 
 /*