[HIP] Support `-fgpu-default-stream`

Introduce -fgpu-default-stream={legacy|per-thread} option to
support per-thread default stream for HIP runtime.

When -fgpu-default-stream=per-thread, HIP kernels are
launched through hipLaunchKernel_spt instead of
hipLaunchKernel. Also HIP_API_PER_THREAD_DEFAULT_STREAM=1
is defined by the preprocessor to enable other per-thread stream
API's.

Reviewed by: Artem Belevich

Differential Revision: https://reviews.llvm.org/D120298