Update hipcc to use HIP_COMPILER variable
Update hipcc to use HIP_COMPILER variable

This will update the hipcc script so that it will use HIP_PLATFORM, HIP_COMPILER, and HIP_RUNTIME variables correctly based on updated hipconfig file.

Removing HIP_PLATFORM=clang, only use hcc and nvcc for now. HIP_PLATFORM should eventually switch to amd or nvcc.

Change-Id: Ie527d07a1add974cc3aab8e9f61b0518147d7602

ROCm-Developer-Tools/HIP,ROCm-Developer-Tools/HIP,ROCm-Developer-Tools/HIP,ROCm-Developer-Tools/HIP,ROCm-Developer-Tools/HIP