[HIP] Make `--offload-add-rpath` alias of `-frtlib-add-rpath`

HIP runtime is the language runtime of HIP. When users need
to specify rpath, they usually need to specify rpath for
both compiler-rt and HIP runtime. It seems redundant
to have separate options. Therefore make --offload-add-rpath
an alias to -frtlib-add-rpath.

Reviewed by: Fangrui Song, Artem Belevich

Differential Revision: https://reviews.llvm.org/D145393