Run rb_gc_before_fork after before_exec

before_exec stops the timer thread, which requires locking the Ractor
scheduler lock. This may deadlock if rb_gc_before_fork locks the VM.

Author: peterzhu2118

process.c

@@ -1575,8 +1575,8 @@ after_exec(void)
 static void
 before_fork_ruby(void)
 {
-    rb_gc_before_fork();
     before_exec();
+    rb_gc_before_fork();
 }
 
 static void