[Driver][SYCL][NewOffload] Fix arch settings for nvptx and amd

[mdtoguchi]

When compiling for -fsycl-targets values of nvptx64-nvidia-cuda and amdgcn-amd-gpu, the default arch behaviors were not applied to the compilation. Updates to do the following:

• Add default of sm_50 for nvptx64 if not provided
• Emit diagnostic if no arch provided for amd
• Parse -Xsycl-backend-target for offload-arch values

[mdtoguchi]

@intel/llvm-gatekeepers, This should be ready for merge - please take a look.

[asudarsa]

We should really be not running CI testing for comment updates...:-(

[martygrant]

@mdtoguchi this is awaiting approval from @srividya-sundaram

[asudarsa]

We have approval from a driver team reviewer, This change unblocks development by fixing a large number of test failures. So, I ill merge it now. @srividya-sundaram, please feel free to reach out to me or @mdtoguchi for any post-commit changes if required.

Thanks Sincerely

[srividya-sundaram]

Sorry, I missed the notification for approval request.