Fix MachO's getRelocationAdditionalInfo.

It was returning the loaded address of the section containing the relocation,
which really doesn't seem to be the intent of this function.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@179255 91177308-0d34-0410-b5e6-96231b3b80d8
Add MachO-x86-64 tests.

The object was already checked in, but was not being tested.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@179256 91177308-0d34-0410-b5e6-96231b3b80d8