[AMDGPU][Libomptarget] Collect allocatable memory pools using HSA

The logic is almost similar to that of system.cpp with one change that
instead of adding all the memory pools to a device struct it only
keeps a single pool. The existing approach also always allocated memory on
the first HSA pool found for a GPU.

This depends on D104691. The goal of this series of patches is to remove
_atl_machine global. The next patch will drop g_atl_machine entirely.

Reviewed By: JonChesterfield

Differential Revision: https://reviews.llvm.org/D104695