[AMDGPU] Always pass `-mcpu` to the `lld` linker

Currently, AMDGPU more or less only supports linking with LTO. If the
user does not either pass `-flto` or `-Wl,-plugin-opt=mcpu=` manually
linking will fail because the architecture's aren't compatible. THis
patch simply passes `-mcpu` by default if it was specified. Should be a
no-op if it's not actually used.

Reviewed By: yaxunl

Differential Revision: https://reviews.llvm.org/D153909