Refactor '-ldl' handling for libgomp proper and plugins

Instead of implicit global 'LIBS="-ldl $LIBS"' via 'AC_CHECK_LIB', make
'-ldl' explicit for libgomp proper, and clean up 'PLUGIN_GCN_LIBS',
'PLUGIN_NVPTX_LIBS' accordingly.

	libgomp/
	* Makefile.am (libgomp_la_LIBADD): Initialize.
	* plugin/configfrag.ac (DL_LIBS): New.
	(PLUGIN_GCN_LIBS): Remove.
	(PLUGIN_NVPTX_LIBS): Don't set in the 'PLUGIN_NVPTX_DYNAMIC' case.
	* plugin/Makefrag.am (libgomp_la_LIBADD)
	(libgomp_plugin_gcn_la_LIBADD): Consider '$(DL_LIBS)'.
	(libgomp_plugin_nvptx_la_LIBADD) <PLUGIN_NVPTX_DYNAMIC>: Likewise.
	* Makefile.in: Regenerate.
	* config.h.in: Likewise.
	* configure: Likewise.
	* testsuite/Makefile.in: Likewise.