arg-pp-main, and allow for wrapper-specific hooks to extend the
pre-processing through _WRAP_ARG_PP.<wrappee>. Move the Darwin
GCC-related preprocessing into arg-pp-darwin-gcc, and create a
arg-pp-mipspro-cc that understands -LANG:<feature> and -LIST:<feature>
so that they're not confused with the usual -Ldir options.
