Allow a user of libLTO to specify the full pathname of the gcc executable to

Allow a user of libLTO to specify the full pathname of the gcc executable to
run when assembling.

Wire this up to the gold plugin. You can now pass --plugin-opt gcc=/foo/bar/gcc
and it will run that gcc instead of looking for it on the path.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@70490 91177308-0d34-0410-b5e6-96231b3b80d8