Use the correct preprocessor macro for detecting riscv64.

Use a preprocessor macro which is defined by the compiler for detecting
riscv64.

gen/ioctl/main.c

@@ -35,8 +35,8 @@ int main(void) {
     printf("#ifdef __powerpc__\n");
 #elif defined(__powerpc64__)
     printf("#ifdef __powerpc64__\n");
-#elif defined(__riscv) && __WORDSIZE == 64
-    printf("#if defined(__riscv) && __WORDSIZE == 64\n");
+#elif defined(__riscv) && __riscv_xlen == 64
+    printf("#if defined(__riscv) && __riscv_xlen == 64\n");
 #else
 #error "unimplemented architecture"
 #endif

gen/modules/ioctl.h

@@ -7570,7 +7570,7 @@
 #define ACRN_IOCTL_DESTROY_IOREQ_CLIENT 0x2000a234ul
 #define ACRN_IOCTL_VM_INTR_MONITOR 0x8008a224ul
 #endif
-#if defined(__riscv) && __WORDSIZE == 64
+#if defined(__riscv) && __riscv_xlen == 64
 #define FIONREAD 0x541bul
 #define FIONBIO 0x5421ul
 #define FIOCLEX 0x5451ul