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).

gcc/ChangeLog:

	* omp-low.cc (omp_runtime_api_call): Added target_memcpy_async and
	target_memcpy_rect_async to omp_runtime_apis array.

libgomp/ChangeLog:

	* libgomp.map: Added omp_target_memcpy_async and
	omp_target_memcpy_rect_async.
	* libgomp.texi: Both functions are now supported.
	* omp.h.in: Added omp_target_memcpy_async and
	omp_target_memcpy_rect_async.
	* omp_lib.f90.in: Added interfaces for both new functions.
	* omp_lib.h.in: Likewise.
	* target.c (ialias_redirect): Added for GOMP_task.
	(omp_target_memcpy): Restructured into check and copy part.
	(omp_target_memcpy_check): New helper function for omp_target_memcpy and
	omp_target_memcpy_async that checks requirements.
	(omp_target_memcpy_copy): New helper function for omp_target_memcpy and
	omp_target_memcpy_async that performs the memcpy.
	(omp_target_memcpy_async_helper): New helper function that is used in
	omp_target_memcpy_async for the asynchronous task.
	(omp_target_memcpy_async): Added.
	(omp_target_memcpy_rect): Restructured into check and copy part.
	(omp_target_memcpy_rect_check): New helper function for
	omp_target_memcpy_rect and omp_target_memcpy_rect_async that checks
	requirements.
	(omp_target_memcpy_rect_copy): New helper function for
	omp_target_memcpy_rect and omp_target_memcpy_rect_async that performs
	the memcpy.
	(omp_target_memcpy_rect_async_helper): New helper function that is used
	in omp_target_memcpy_rect_async for the asynchronous task.
	(omp_target_memcpy_rect_async): Added.
	* task.c (ialias): Added for GOMP_task.
	* testsuite/libgomp.c-c++-common/target-memcpy-async-1.c: New test.
	* testsuite/libgomp.c-c++-common/target-memcpy-async-2.c: New test.
	* testsuite/libgomp.c-c++-common/target-memcpy-rect-async-1.c: New test.
	* testsuite/libgomp.c-c++-common/target-memcpy-rect-async-2.c: New test.
	* testsuite/libgomp.fortran/target-memcpy-async-1.f90: New test.
	* testsuite/libgomp.fortran/target-memcpy-async-2.f90: New test.
	* testsuite/libgomp.fortran/target-memcpy-rect-async-1.f90: New test.
	* testsuite/libgomp.fortran/target-memcpy-rect-async-2.f90: New test.