Attempt to register OpenMP pinned memory using a device instead of 'mlock'

Attempt to register OpenMP pinned memory using a device instead of 'mlock'

Implemented for nvptx offloading via 'cuMemHostRegister'.  This means: (a) not
running into 'mlock' limitations, and (b) the device is aware of this and may
optimize host <-> device memory transfers.

This re-works og12 commit ab7520b3b4cd9fdabfd63652badde478955bd3b5
"libgomp: pinned memory".

	include/
	* cuda/cuda.h (cuMemHostRegister, cuMemHostUnregister): New.
	libgomp/
	* config/linux/allocator.c (linux_memspace_alloc)
	(linux_memspace_free, linux_memspace_realloc): Attempt to register
	OpenMP pinned memory using a device instead of 'mlock'.
	* libgomp-plugin.h (GOMP_OFFLOAD_register_page_locked)
	(GOMP_OFFLOAD_unregister_page_locked): New.
	* libgomp.h (gomp_register_page_locked)
	(gomp_unregister_page_locked): New
	(struct gomp_device_descr): Add 'register_page_locked_func',
	'unregister_page_locked_func'.
	* plugin/cuda-lib.def (cuMemHostRegister_v2, cuMemHostRegister)
	(cuMemHostUnregister): New.
	* plugin/plugin-nvptx.c (GOMP_OFFLOAD_register_page_locked)
	(GOMP_OFFLOAD_unregister_page_locked): New.
	* target.c (gomp_register_page_locked)
	(gomp_unregister_page_locked): New.
	(gomp_load_plugin_for_device): Handle 'register_page_locked',
	'unregister_page_locked'.
	* testsuite/libgomp.c/alloc-pinned-1.c: Adjust.
	* testsuite/libgomp.c/alloc-pinned-2.c: Likewise.
	* testsuite/libgomp.c/alloc-pinned-3.c: Likewise.
	* testsuite/libgomp.c/alloc-pinned-4.c: Likewise.
	* testsuite/libgomp.c/alloc-pinned-5.c: Likewise.
	* testsuite/libgomp.c/alloc-pinned-6.c: Likewise.