Summary:
This was a lot of code that was only used for upstream LLVM builds of
AMDGPU offloading. We have a generic and fast `malloc` in `libc` now so
just use that. Simplifies code, can be added back if we start providing
alternate forms but I don't think there's a single use-case that would
justify it yet.

Follow-up from #162652

---------

Co-authored-by: Michael Klemm <michael.klemm@amd.com>

Adds omp_target_is_accessible routine.
Refactors common code from omp_target_is_present to work for both
routines.

---------

Co-authored-by: Shilei Tian <i@tianshilei.me>

On Windows, for a reason I don't fully understand boolean bits get extra
padding (even when asking for packed structures) in the structures that
messes the offsets between the compiler and the runtime.

Also, "weak" works differently on Windows than Linux (i.e., the "local"
routine has preference) which causes it to crash as we don't really have
an alternate implementation of __kmpc_omp_wait_deps. Given this, it
doesn't make sense to mark it as "weak" for Linux either.

Summary:
This was originally added in as a hack to work around CUDA's limitation
on allocation. The `libc` implementation now isn't even used for CUDA so
this code is never hit. Even if this case, this code never truly worked.

A true solution would be to use CUDA's virtual memory API instead to
allocate 2MiB slabs independenctly from the normal memory management
done in the stream.

This change adds support for saving full contents of attached Fortran
descriptors, and not just their pointee address, in the shadow-pointer
table.

With this, we now support:
* comparing full contents of descriptors to check whether a previous
shadow-pointer entry is stale;
* restoring the full contents of descriptors

And with that, we can now use ATTACH map-types (added in #149036) for
mapping Fortran pointer/allocatable arrays, and array-sections on them.
e.g.:

```f90
  integer, allocatable :: x(:)
  !$omp target enter data map(to: x(:))
```

as:

```
  void* addr_of_pointee = allocated(x) ? &x(1) : nullptr;
  int64_t sizeof_pointee = allocated(x) ? sizeof(x(:)) : 0

  addr_of_pointee,    addr_of_pointee, sizeof_pointee,     TO
  addr_of_descriptor, addr_of_pointee, size_of_descriptor, ATTACH
```

Summary:
This operation is done every time we load a binary, this behavior should
be moved into OpenMP since it concerns an OpenMP specific data struct.
This is a little messy, because ideally we should only be using public
APIs, but more can be extracted later.

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>