AMDGPU: Use lld as the linker again

Summary:
Now that LLVM is emitting version 2 of the AMD code object, we can
start using lld again for linking instead of our custom tool.

Reviewers: arsenm, kzhuravl

Subscribers: rafael, cfe-commits

Differential Revision: http://reviews.llvm.org/D19952

llvm-svn: 268648