Always use the vendored libgit2, unless something else is specified.
Allow to cross compile x64 binaries.
Make sure we use the correct `strip` binary.
Compiling libgit2 should be a pre-requisite of the rugged binary.

Previously, the libgit2 binary would not be built when calling the
`rake cross native gem` tasks.
Make sure we always remove libgit2 build artifacts.

The `clean:libgit2` task was only executed once even when building for
multiple platforms, which would fail.
Fix the path the libgit2 static library is copied to.