This patch is a follow up on https://reviews.llvm.org/D81627.
In addition to default -fno-gpu-rdc case, this patches let
HIP toolchain not use llvm-link/opt/llc to link device code
for -fgpu-rdc case. Instead, uses standard lto.
This will eliminate some redundant optimizations and speed
up the compilation/linking.
Differential Revision: https://reviews.llvm.org/D81861
Yaxun (Sam) Liu