Simplify OpenACC 'no_create' clause implementation

Simplify OpenACC 'no_create' clause implementation

For 'OFFSET_INLINED', 'gomp_map_val' does the right thing, and we may then
simplify the device plugins accordingly.

This is a follow-up to
Subversion r279551 (Git commit a6163563f2ce502bd4ef444bd5de33570bb8eeb1)
"Add OpenACC 2.6's no_create",
Subversion r279622 (Git commit 5bcd470bf0749e1f56d05dd43aa9584ff2e3a090)
"Use gomp_map_val for OpenACC host-to-device address translation".

	libgomp/
	* target.c (gomp_map_vars_internal): Use 'OFFSET_INLINED' for
	'GOMP_MAP_IF_PRESENT'.
	* plugin/plugin-gcn.c (gcn_exec, GOMP_OFFLOAD_openacc_exec)
	(GOMP_OFFLOAD_openacc_async_exec): Adjust.
	* plugin/plugin-nvptx.c (nvptx_exec, GOMP_OFFLOAD_openacc_exec)
	(GOMP_OFFLOAD_openacc_async_exec): Likewise.
	* testsuite/libgomp.oacc-c-c++-common/no_create-1.c: Add 'async'
	testing.
	* testsuite/libgomp.oacc-c-c++-common/no_create-2.c: Likewise.