Libomptarget contains its own constructor and destructor for
initializing and finalizing state. This creates some implicit ordering
requirements for the construction. The host already calls into
libomptarget to initialize the libraries. This patch changes this to
also initialize the plugin manager used in libomptarget. We do the init
/ deinit if this is the first / last user to require the plugins. If the
plugin manager is not initialized we exit as we cannot perform
offloading and fall back to the host.
One snag in this design is the use of the register_requires function
which may attempt to initialize the flags before the plugin has been
initialized as it is also a constructor called from the host. I
attempted to solve this by registering them later if not initialized,
but that is not an ideal solution.