Hi,

This patch adds two new OpenMP runtime routines: omp_target_memcpy_async and
omp_target_memcpy_rect_async. Both functions are introduced in OpenMP 5.1 as
asynchronous variants of omp_target_memcpy and omp_target_memcpy_rect.

In contrast to the synchronous variants, the asynchronous functions have two
additional function parameters to allow the specification of task dependences:

        int depobj_count
        omp_depend_t *depobj_list

        integer(c_int), value :: depobj_count
        integer(omp_depend_kind), optional :: depobj_list(*)

The implementation splits the synchronous functions into two parts: (a) check
and (b) copy. Then (a) is used in the asynchronous functions for the sequential
part, and the actual copy process (b) is executed in a new created task. The
sequential part (a) takes into account the requirements for the return values:

"The routine returns zero if successful. Otherwise, it returns a non-zero
value." (omp_target_memcpy_async, OpenMP 5.1 spec, section 3.8.7)

"An application can determine the number of inclusive dimensions supported by an
implementation by passing NULL pointers (or C_NULL_PTR, for Fortran) for both
dst and src. The routine returns the number of dimensions supported by the
implementation for the specified device numbers. No copy operation is
performed." (omp_target_memcpy_rect_async, OpenMP 5.1 spec, section 3.8.8)

Due to asynchronicity an error is thrown if the asynchronous memcpy is not
successful (in contrast to the synchronous functions which use a return
value unequal to zero).

The patch was tested on x86_64-linux with nvptx and amdgcn offloading and with
PowerPC with nvptx offloading. All with no regressions.

Marcel
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