[AMDGPU] Support -mcpu=native for OpenCL

When -mcpu=native is specified, try detecting GPU
on the system by using amdgpu-arch tool. If it
fails to detect GPU, emit an error about GPU
not detected. If multiple GPUs are detected,
use the first GPU and emit a warning.

Reviewed by: Matt Arsenault, Fangrui Song

Differential Revision: https://reviews.llvm.org/D154531