Summary:
Currently, we register images into a linear table according to the
logical OpenMP device identifier. We then initialize all of these images
as one block. This logic requires that images are compatible with *all*
devices instead of just the one that it can run on. This prevents us
from running on systems with heterogeneous devices (i.e. image 1 runs on
device 0 image 0 runs on device 1).

This patch reworks the logic by instead making the compatibility check a
per-device query. We then scan every device to see if it's compatible
and do it as they come.

Summary:
There was a bug here where we would initialize the plugin multiple times
when there were multiple images. Fix it by putting the `is_initliaized`
check later.

Summary:
Initializing the plugins requires initializing the runtime like CUDA or
HSA. This has a considerable overhead on most platforms, so we should
only actually initialize a plugin if it is needed by any image that is
loaded.

Summary:
This isn't `libomptarget` anymore, and these messages were always
unnecessary because no other project uses these prefixed messages. The
effect of this is that no longer will the logs have `LIBOMPTARGET --` in
front of everything. We have a message stating when we start building
the offload project so it'll still be trivial to find.

Summary:
This patch cleans up the training whitespace in a bunch of tests and
CMake files. Most just in preparation for other cleanups.

Summary:
No other project has these in the CMake itself, and they're wildly
inconsistent even within the project. These don't really add anything so
I think they should be removed.

Summary:
A runtime library should not have global constructors. Everything is now
expected to go through the init methods. This patch ensures that global
constructors will not accidentally be introduced.

Summary:
Currently this is only used for the zero-copy handling. However, this
can easily be moved into `libomptarget` so that we do not need to bother
setting the requires flags in the plugin. The advantage here is that we
no longer need to do this for every device redundently. Additionally,
these requires flags are specifically OpenMP related, so they should
live in `libomptarget`.

Summary:
These are lots of random warnings due to inconsistent initialization or
signedness.