This patch introduces libomptarget support for the ATTACH map-type,
which can be used to implement OpenMP conditional compliant pointer
attachment, based on whether the pointer/pointee is newly mapped on a
given construct.

For example, for the following:

```c
  int *p;
  #pragma omp target enter data map(p[1:10])
```

The following maps can be emitted by clang:
```
  (A)
  &p[0], &p[1], 10 * sizeof(p[1]), TO | FROM
  &p, &p[1], sizeof(p), ATTACH
```

Without this map-type, these two possible maps could be emitted by
clang:
```
  (B)
  &p[0], &p[1], 10 * sizeof(p[1]), TO | FROM

  (C)
  &p, &p[1], 10 * sizeof(p[1]), TO | FROM | PTR_AND_OBJ
````

(B) does not perform any pointer attachment, while (C) also maps the
pointer p, which are both incorrect.

In terms of implementation, maps with the ATTACH map-type are handled
after all other maps have been processed, as it requires knowledge of
which new allocations happened as part of the construct. As per OpenMP
5.0, an attachment should happen only when either the pointer or the
pointee was newly mapped while handling the construct.

Maps with ATTACH map-type-bit do not increase/decrease the ref-count.

With OpenMP 6.1, `attach(always/never)` can be used to force/prevent
attachment. For `attach(always)`, the compiler will insert the ALWAYS
map-type, which would let libomptarget bypass the check about one of the
pointer/pointee being new. With `attach(never)`, the ATTACH map will not
be emitted at all.

The size argument of the ATTACH map-type can specify values greater than
`sizeof(void*)` which can be used to support pointer attachment on
Fortran descriptors. Note that this also requires shadow-pointer
tracking to also support them. That has not been implemented in this
patch.

This was worked upon in coordination with Ravi Narayanaswamy, who has
since retired. Happy retirement, Ravi!

---------

Co-authored-by: Alex Duran <alejandro.duran@intel.com>

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).

Summary:
The name `src` is confusing when combined with the plugins and the newly
added `liboffload`.