Refactor the cross ruby code even more.

- CrossRuby#tool returns the name of a binutils command.
- RUBY_CC_VERSION is built from .cross_ruby.
- Properly escape strings using Regexp.quote and String#shelljoin.
Merge branch 'larskanis-cleanup-mingw-build' into refactor-cross_rubies

Conflicts:
	Rakefile
Strip original so files before copy to avoid complex dependencies.