Add a testcase for PR ld/17973

Add a testcase for PR ld/17973

ld/

	PR ld/17973
	* testplug2.c (allsymbolsread_silent): New.
	(set_register_hook): Handle allsymbolsreadsilent.
	(onall_symbols_read): Skip a message if allsymbolsread_silent is
	TRUE.

ld/testsuite/

	PR ld/17973
	* ld-plugin/plugin.exp (regassilent): New.
	Compile tmpdir/dummy.s and ld-plugin/pr17973.s.
	(plugin_tests): Add a test for PR ld/17973.
	* ld-plugin/pr17973.d: New file.
	* ld-plugin/pr17973.s: Likewise.