Start adding some tests for the gold plugin.

These are only used when the 'ld' in the path is gold and the plugin has
been built, but it is already a start to make sure we don't regress features
that cannot be tested with llvm-lto.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@214058 91177308-0d34-0410-b5e6-96231b3b80d8