AMDGPU plugin equivalent of D95155, build without HSA installed locally

Compiles a new file, plugins/amdgpu/dynamic_hsa/hsa.cpp, to an object file that
exposes the same symbols that the plugin presently uses from hsa. The object
file contains dlopen of hsa and cached dlsym calls. Also provides header files
corresponding to the subset that is used.

This is behind a feature flag, LIBOMPTARGET_FORCE_DLOPEN_LIBHSA, default off.
That allows developers to build against the dlopen/dlsym implementation, e.g.
while testing this mode.

Enabling by default will cause this plugin to build on a wider variety of
machines than it does at present so may break some CI builds. That risk can
be minimised by reviewing the header dependencies of the library and ensuring
it doesn't use any libraries that are not already used by libomptarget.

Separating the implementation from enabling by default in case the latter needs
to be rolled back after wider CI results.

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D106559