[Clang] Respect `-L` options when compiling directly for AMDGPU

The AMDGPU linker is `lld`, which has full support for standard features
like static libraries. Previously the AMDGPU toolchain did not forward
`-L` arguments so we could not tell it where to find certain libraries.
This patch simply forwards it like the other toolchains.

Reviewed By: yaxunl, MaskRay

Differential Revision: https://reviews.llvm.org/D150013