Add a spiderwasm_prologue_words setting.

This makes the details of the spiderwasm prologue configurable so it is
easier to modify SpiderMonkey without having to change Cretonne.

Create a stack object representing the SpiderMonkey prologue words
before calculating the stack layout so they won't be overwritten by
Cretonne's stack objects.

lib/cretonne/src/settings.rs

@@ -323,10 +323,12 @@ mod tests {
                     is_compressed = false\n\
                     enable_float = true\n\
                     enable_simd = true\n\
-                    enable_atomics = true\n"
+                    enable_atomics = true\n\
+                    spiderwasm_prologue_words = 0\n"
         );
         assert_eq!(f.opt_level(), super::OptLevel::Default);
         assert_eq!(f.enable_simd(), true);
+        assert_eq!(f.spiderwasm_prologue_words(), 0);
     }
 
     #[test]