[libomptarget][amdgpu] Precisely manage hsa lifetime

The hsa library must be initialized before any calls into it and
destructed after the last call into it. There have been a number of bugs in
this area related to member variables which would like to use raii to manage
resources acquired from hsa.

This patch moves the init/shutdown of hsa into a class, such that when used as
the first member variable (could be a base), the lifetime of other member
variables are reliably scoped within it. This will allow other classes to use
raii reliably when used as member variables within the global.

Reviewed By: pdhaliwal

Differential Revision: https://reviews.llvm.org/D109512