contrib: lame: Don't use -mtune=native/-march=native

When building a distribution package, tuning specifically for the system building it is pointless. (If we want to tune specifically for something else than the compiler's default, we should specify that and not rely on it being implied from the system doing the compilation.)

When cross compiling VLC for x86 from a non-x86 system, -mtune=native and -march=native cause compiler errors (in practice, e.g. for i686-w64-mingw32 targets built on aarch64, GCC doesn't mind -mtune=native but errors out for -march=native, while Clang errors out for -mtune=native too).