[libomptarget] Build amdgpu plugin by default

[libomptarget] Build amdgpu plugin by default

This will build the amdgpu plugin if cmake is able to find the hsa
runtime library, which will be the case if rocm is installed or if
the hsa library has been installed somewhere cmake looks.

Reviewed By: tianshilei1992

Differential Revision: https://reviews.llvm.org/D98654