[AMDGCN] Update search path for device libraries

- Add support for finding device libraries in new ROCm directory
structure
- Simplify and remove the handling of legacy ROCm directory structure

Change-Id: I04da3bc9da85ced4b56b0225efb6b94448b8c5a1

Reviewed By: yaxunl

Differential Revision: https://reviews.llvm.org/D140315