fix logic for finding nvcc even if clang exists

[acxz]

Issue: if a system has both nvcc and clang++ (compiled w/o amd) then the HIP-PLATFORM autodetects to amd, instead of nvidia.

this PR fixes it by only checking for clang++ compiled with amd with the provided alias of amdclang++.

Follow up of https://github.com/ROCm-Developer-Tools/HIP/pull/2623

Fixes: https://github.com/ROCm-Developer-Tools/HIP/issues/1650

Fixes comment: https://github.com/ROCm-Developer-Tools/HIP/issues/2256#issuecomment-818801109

cc: @mangupta