Add wrappers for strip commands so that files are copied to the local drive before stripped.

Stripping files on a local folder instead of a Virtualbox shared folder
is needed to work around this bug: https://www.virtualbox.org/ticket/8463

Without that wrapper strip fails with something like this:
```
vagrant@precise64:/vagrant/nokogiri$ i686-w64-mingw32-strip -S tmp/x86-mingw32/stage/lib/nokogiri/2.0/nokogiri.so
i686-w64-mingw32-strip:tmp/x86-mingw32/stage/lib/nokogiri/2.0/stBxc8W1: Protocol error
```