Summary:
This check is unnecessarily restrictive and currently incorrectly fires
for any size less than eight bytes. Just remove it, we do sanity checks
elsewhere and at some point need to trust the ABI.

This sprinkles a few mutexes around the plugin interface so that the
olLaunchKernel CTS test now passes when ran on multiple threads.

Part of this also involved changing the interface for device synchronise
so that it can optionally not free the underlying queue (which
introduced a race condition in liboffload).

Rather than having every "enqueue"-type function have an output pointer
specifically for an output event, just provide an `olCreateEvent`
entrypoint which pushes an event to the queue.

For example, replace:
```cpp
olMemcpy(Queue, ..., EventOut);
```
with
```cpp
olMemcpy(Queue, ...);
olCreateEvent(Queue, EventOut);
```

This more closely matches the nomenclature used by CUDA, AMDGPU and
the plugin interface.

`olGetKernel` has been replaced by `olGetSymbol` which accepts a
`Kind` parameter. As well as loading information about kernels, it
can now also load information about global variables.

In the future, we want `ol_symbol_handle_t` to represent both kernels
and global variables The first step in this process is a rename and
promotion to a "typed handle".

Adds two "launch kernel" tests for lib offload, one testing that
global memory works and persists between different kernels, and one
verifying that `[[gnu::constructor]]` works correctly.

Since we now have tests that contain multiple kernels in the same
binary, the test framework has been updated a bit.

This is a three element x, y, z size_t vector that can be used any place
where a 3D vector is required. This ensures that all vectors across
liboffload are the same and don't require any resizing/reordering
dances.