Convert tools/lib32/build.sh into world connectable hooks. This still
rates pretty high on the "hack!" scale, but it works for me. Adding
-DWANT_LIB32 to the world build command line, or 'WANT_LIB32=yes' to
/etc/make.conf will include the 32 bit libraries with the build.

I have not made this default behavior. Cross compiling this stuff is an
adventure I have not investigated.

This is still a WIP. We needed this at work so that we could install from
a readonly obj tree - lib32/build.sh wasn't up to that.