Hi,

This patch adds the OpenMP runtime routine "omp_target_is_accessible" which was
introduced in OpenMP 5.1 (specification section 3.8.4):

"The omp_target_is_accessible routine tests whether host memory is accessible
from a given device."

"This routine returns true if the storage of size bytes starting at the address
given by ptr is accessible from device device_num. Otherwise, it returns false."

"The value of ptr must be a valid host pointer or NULL (or C_NULL_PTR, for
Fortran). 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()."

"When called from within a target region the effect is unspecified."

Currently, the only way of accessing host memory on a non-host device is via
shared memory. This will change with unified shared memory (usm) that was
recently submitted but not yet approved/committed. A follow-up patch for
omp_target_is_accessible is planned considering usm when available. The current
patch handles the basic implementation for C/C++ and Fortran and includes
comments pointing to usm.

Although not explicitly specified in the OpenMP 5.1 standard, the implemented
function returns "true" if the given device_num is equal to
"omp_get_num_devices" (i.e. the host) as it is expected that host memory can be
accessed from the host device.

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

Marcel
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