This is based on the decision The NetBSD Foundation made in 2008 to
do so, which was already applied to src.
This change has been applied to code which is likely not in other
repositories.
ok board@, reviewed by riastradh@
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.
