libgomp: Enable USM for some nvptx devices

libgomp: Enable USM for some nvptx devices

A few high-end nvptx devices support the attribute
CU_DEVICE_ATTRIBUTE_PAGEABLE_MEMORY_ACCESS; for those, unified shared
memory is supported in hardware. This patch enables support for those -
if all installed nvptx devices have this feature (as the capabilities
are per device type).

This exposes a bug in gomp_copy_back_icvs as it did before use
omp_get_mapped_ptr to find mapped variables, but that returns
the unchanged pointer in cased of shared memory. But in this case,
we have a few actually mapped pointers - like the ICV variables.
Additionally, there was a mismatch with regards to '-1' for the
device number as gomp_copy_back_icvs and omp_get_mapped_ptr count
differently. Hence, do the lookup manually.

include/ChangeLog:

	* cuda/cuda.h (CU_DEVICE_ATTRIBUTE_PAGEABLE_MEMORY_ACCESS): Add.

libgomp/ChangeLog:

	* libgomp.texi (nvptx): Update USM description.
	* plugin/plugin-nvptx.c (GOMP_OFFLOAD_get_num_devices):
	Claim support when requesting USM and all devices support
	CU_DEVICE_ATTRIBUTE_PAGEABLE_MEMORY_ACCESS.
	* target.c (gomp_copy_back_icvs): Fix device ptr lookup.
	(gomp_target_init): Set GOMP_OFFLOAD_CAP_SHARED_MEM is the
	devices supports USM.