Remove checks for constructors that might have been eliminated by GCC.

Remove checks for constructors that might have been eliminated by GCC.

GCC 6 does not generate constructors for two of gold's test cases.
This patch simply removes the checks for them.

gold/
	PR 19751
	* testsuite/Makefile.am (retain_symbols_file_test): Remove check
	for constructor.
	* testsuite/Makefile.in: Regenerate.
	* testsuite/dynamic_list.sh: Likewise.
	* testsuite/retain_symbols_file_test.sh: Likewise.