Driver: Factor gold support out into a function

AddGoldPlugin() is the same on gnutools and freebsd so avoid the
copy-and-paste duplicate code.

No change in functionality.

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@196096 91177308-0d34-0410-b5e6-96231b3b80d8