Output "warning:" or "error:" in plugin messages

Output "warning:" or "error:" in plugin messages

When plugin generates LDPL_WARNING, LDPL_FATAL and LDPL_ERROR messages,
linker should display "warning:" or "error:" in plugin messages like
regular linker messages.

ld/

	* plugin.c (message): Output "warning:" for LDPL_WARNING. Output
	"error:" for LDPL_FATAL and LDPL_ERROR.
	* testplug2.c (parse_option): Handle fatal, error and warning.

ld/testsuite/

	* ld-plugin/plugin-27.d: New.
	* ld-plugin/plugin-28.d: Likewise.
	* ld-plugin/plugin-29.d: Likewise.
	* plugin.exp (plugin_tests): Add tests for LDPL_FATAL, LDPL_ERROR
	and LDPL_WARNING.