OpenMP, NVPTX: memcpy[23]D bias correction

This patch works around behaviour of the 2D and 3D memcpy operations in
the CUDA driver runtime.  Particularly in Fortran, the "base pointer"
of an array (used for either source or destination of a host/device copy)
may lie outside of data that is actually stored on the device.  The fix
is to make sure that we use the first element of data to be transferred
instead, and adjust parameters accordingly.

2023-10-02  Julian Brown  <julian@codesourcery.com>

libgomp/
	* plugin/plugin-nvptx.c (GOMP_OFFLOAD_memcpy2d): Adjust parameters to
	avoid out-of-bounds array checks in CUDA runtime.
	(GOMP_OFFLOAD_memcpy3d): Likewise.
	* testsuite/libgomp.c-c++-common/memcpyxd-bias-1.c: New test.