[libgomp] Clarify difference between offload target, offload plugin, and OpenACC...

[libgomp] Clarify difference between offload target, offload plugin, and OpenACC device type

	libgomp/
	* plugin/configfrag.ac: Populate and AC_SUBST offload_plugins
	instead of offload_targets, and AC_DEFINE_UNQUOTED OFFLOAD_PLUGINS
	instead of OFFLOAD_TARGETS.
	* target.c (gomp_target_init): Adjust.
	* testsuite/libgomp-test-support.exp.in: Likewise.
	* testsuite/lib/libgomp.exp: Likewise.  Populate
	openacc_device_types_s instead of offload_targets_s_openacc.
	(check_effective_target_openacc_nvidia_accel_selected)
	(check_effective_target_openacc_host_selected): Adjust.
	* testsuite/libgomp.oacc-c++/c++.exp: Likewise.
	* testsuite/libgomp.oacc-c/c.exp: Likewise.
	* testsuite/libgomp.oacc-fortran/fortran.exp: Likewise.
	* Makefile.in: Regenerate.
	* config.h.in: Likewise.
	* configure: Likewise.
	* testsuite/Makefile.in: Likewise.

From-SVN: r269107