GCC47: 4.7.3.

* GCC 4.7 now can be bootstrapped by Clang. Drop fails_with part.
* Remove obsolete ENV.m64 part. GCC 4.7 detects host triplets.
* Apply patches from macports to fix debugging.
* Don't unset __GLIBCXX_DEBUG* as original bug didn't actually do that.
* Make arch and osmajor standalone functions, and construct host triplet
  for build.
* Integrate '--enable-languages' into args.