Exit if YJIT and MJIT are both enabled

YJIT and MJIT can't be running in the same process otherwise they'll
clobber each other.  We should show an error and exit if they're both
enabled.

ruby.c

@@ -1834,13 +1834,18 @@ process_options(int argc, char **argv, ruby_cmdline_options_t *opt)
          */
         rb_warning("-K is specified; it is for 1.8 compatibility and may cause odd behavior");
 
-    if (opt->features.set & FEATURE_BIT(yjit))
-        rb_yjit_init(&opt->yjit);
 #if USE_MJIT
     if (opt->features.set & FEATURE_BIT(jit)) {
         opt->mjit.on = TRUE; /* set mjit.on for ruby_show_version() API and check to call mjit_init() */
     }
 #endif
+    if (opt->features.set & FEATURE_BIT(yjit)) {
+        if (opt->mjit.on) {
+            rb_warn("MJIT and YJIT are both enabled at the same time. Exiting");
+            exit(1);
+        }
+        rb_yjit_init(&opt->yjit);
+    }
     if (opt->dump & (DUMP_BIT(version) | DUMP_BIT(version_v))) {
 #if USE_MJIT
         mjit_opts.on = opt->mjit.on; /* used by ruby_show_version(). mjit_init() still can't be called here. */