[llvm-objcopy] Use the full filename in --add-gnu-debuglink

Summary:
The current implementation was writing the file name without the extension
whereas GNU objcopy writes the full filename. With this change GDB will now
load the .debug file instead of silently ignoring it.

Reviewers: jakehehrlich, jhenderson

Reviewed By: jakehehrlich

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D43474

llvm-svn: 325528