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.
(cherry picked from commit
dcc266796a6afa18eeba1edde81f9eb78e6c83ce)