[Clang] Add options in LTO mode when cross compiling for AMDGPU

The AMDGPU toolchain support directly compiling GPU images using
cross-compilation such as `clang --target=amdgcn-amd-amdhsa foo.c`.
However, when attempting to link bitcode this does not work because the
`-mcpu` options are not forwarded to the linker among others. This patch
simply adds them so that `clang --target=amdgcn-amd-amdhsa foo.c -flto`
works correctly.

Reviewed By: JonChesterfield

Differential Revision: https://reviews.llvm.org/D144505