Metrowerks: set optlevel 3 to max optimization args

According to the Meson documentation, optimization level 3 should
set the highest possible optimization for the compiler in use.
In Metrowerks, this is 'O4,p'. However, Meson's Metrowerks
implementation mapped opt level 3 to '-O3'. This has been fixed.

mesonbuild/compilers/mixins/metrowerks.py

@@ -173,7 +173,7 @@ mwcc_optimization_args: T.Dict[str, T.List[str]] = {
     'g': ['-Op'],
     '1': ['-O1'],
     '2': ['-O2'],
-    '3': ['-O3'],
+    '3': ['-O4,p'],
     's': ['-Os']
 }