Metrowerks assembler should not inherit opt args from mixin

The Metrowerks assembler does not support optimization flags.
However, it received the same opt args as the Metrowerks C and C++
compilers, because it inherits from the 'MetrowerksCompiler' mixin.
This broke builds with opt level higher than 0 that used the Metrowerks
Assembler, as the latter received unsupported args. This is now fixed.

mesonbuild/compilers/asm.py

@@ -306,6 +306,9 @@ class MetrowerksAsmCompiler(MetrowerksCompiler, Compiler):
     def get_crt_compile_args(self, crt_val: str, buildtype: str) -> T.List[str]:
         return []
 
+    def get_optimization_args(self, optimization_level: str) -> T.List[str]:
+        return []
+
     def get_pic_args(self) -> T.List[str]:
         return []