[Committed] Fix the gcc.dg/plugin/plugindir*.c tests for darwin

[Dominique d'Humières]

Committed on trunk as obvious

        * gcc.dg/plugin/plugindir1.c: Adjust dg-prune-output for Darwin.
        * gcc.dg/plugin/plugindir2.c: Likewise.
        * gcc.dg/plugin/plugindir3.c: Likewise.
        * gcc.dg/plugin/plugindir4.c: Likewise.

Dominique