Hi,

This patch adds the OpenMP runtime routine "omp_get_mapped_ptr" which was
introduced in OpenMP 5.1 (specification section 3.8.11):

"The omp_get_mapped_ptr routine returns the device pointer that is associated
with a host pointer for a given device."

"The device_num argument must be greater than or equal to zero and less than or
equal to the result of omp_get_num_devices()."

"A call to this routine for a pointer that is not NULL (or C_NULL_PTR, for
Fortran) and does not have an associated pointer on the given device results in
a NULL pointer."

"The routine returns NULL (or C_NULL_PTR, for Fortran) if unsuccessful.
Otherwise it returns the device pointer, which is ptr if device_num is the value
returned by omp_get_initial_device()."

Implementation and tests were added for C/C++ and Fortran.

There is a small inconvenience considering zero-length arrays as list items of
the "target map" construct: it seems that zero-length arrays are not associated
correctly there, such that omp_get_mapped_ptr returns NULL instead of the
associated device pointer - in contrast to the situation where a device pointer
is associated with the host pointer via omp_target_associate_ptr.
However, the result for omp_get_mapped_ptr is consistent with
omp_target_is_present (which returns 0, i.e. "not present") in this situation.

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

Marcel
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