[HIP] Use -mlink-builtin-bitcode to link device library

Use -mlink-builtin-bitcode instead of llvm-link to link
device library so that device library bitcode and user
device code can be compiled in a consistent way.

This is the same approach used by CUDA and OpenMP.

Differential Revision: https://reviews.llvm.org/D60513

llvm-svn: 358290