[SYCL] Enable AMD GPU support.

[malixian]

Enable AMG GPU for DPC++. To support this feature, We did two parts of development, namely the compilation tool chain and the runtime. We have implemented rocm-plugin refer to cuda-plugin. Many test cases in this project have passed, but there are still some problems which can be roughly divided into three areas:
First, some errors occurred at link time. like all-pairs-disance-sycl case occurred error: undefined hidden symbol: __spirv_ControlBarrier will appear during the lld link period due to use barrier(access::fence_space::local_space). Similarly, the undefined__spirv_SubgroupShuffleINTEL error occurs when the cl::sycl::atomic keyword is used at that time.
Second, some errors occurred at runime. the program will core dump when calling hipMemcpyDtoHAsync API due to allocating memory size is too large. But cuda does not have this problem.
Finally, calculation accuracy problem. Currently, the calculation of float type kernel functions is inaccurate, and there is no problem with int type testing. We haven't figured out where the problem is.
We will keep track of the above issues.

[bader]

@malixian, one note.
As of today, we don't have AMD GPU HW in our CI system, so we won't be able to validate that other contributions do not break ROCM support.
I think we should decide ASAP, how ROCM support is going to be verified.

[malixian]

This is indeed a problem.

[alexbatashev]

At least for plugin we can use hip<->cuda source code compatibility

[alexbatashev]

Breaking changes are not allowed. Please, fix backend enum.

[pvchupin]

@malixian, thanks for contribution! Please update documentation, https://github.com/intel/llvm/blob/sycl/sycl/doc/GetStartedGuide.md at least.

[malixian]

@malixian, thanks for contribution! Please update documentation, https://github.com/intel/llvm/blob/sycl/sycl/doc/GetStartedGuide.md at least.

All right.

[malixian]

We update the code and GetStartedGuide.md document for AMDGPU.

[AGindinson]

Per discussion with @bader, adding & committing 3 suggestions (tested locally) to:

1. Apply the Driver LIT patch correctly to pass the checks
2. Remove __SYCL_AMDGCN__ macro due to the similar efforts for __SYCL_NVPTX__ in #3977

[JonChesterfield]

If barrier(access::fence_space::local_space) maps onto the llvm fence instruction, you might be interested in the intrinsic __builtin_amdgcn_fence, https://reviews.llvm.org/D75917

[alexbatashev]

If barrier(access::fence_space::local_space) maps onto the llvm fence instruction, you might be interested in the intrinsic __builtin_amdgcn_fence, https://reviews.llvm.org/D75917

__builtin_amdgcn_s_barrier is probably a better fit for barrier. The global problem, however, is that a lot of things are missing here.