libgomp: Prepare for reverse offload fn lookup

Prepare for reverse-offloading function-pointer lookup by passing
a rev_fn_table argument to GOMP_OFFLOAD_load_image.

The argument will be NULL, unless GOMP_REQUIRES_REVERSE_OFFLOAD is
requested and devices not supported it, are filtered out.
(Up to and including this commit, no non-host device claims such
support and the caller currently always passes NULL.)

libgomp/ChangeLog:

	* libgomp-plugin.h (GOMP_OFFLOAD_load_image): Add
	'uint64_t **rev_fn_table' argument.
	* oacc-host.c (host_load_image): Likewise.
	* plugin/plugin-gcn.c (GOMP_OFFLOAD_load_image): Likewise;
	currently unused.
	* plugin/plugin-nvptx.c (GOMP_OFFLOAD_load_image): Likewise.
	* target.c (gomp_load_image_to_device): Update call but pass
	NULL for now.

liboffloadmic/ChangeLog:

	* plugin/libgomp-plugin-intelmic.cpp (GOMP_OFFLOAD_load_image):
	Add (unused) uint64_t **rev_fn_table argument.