libstdc++: Fix tests that fail with old std::string ABI

libstdc++: Fix tests that fail with old std::string ABI

These two tests have started to fail with the old std::string ABI. The
scan-assembler-not checks fail because they match debug info, not code.

Adding -g0 to the test flags fixes them.

libstdc++-v3/ChangeLog:

	* testsuite/21_strings/basic_string/modifiers/assign/char/move_assign_optim.cc:
	Do not generate debug info.
	* testsuite/21_strings/basic_string/modifiers/assign/wchar_t/move_assign_optim.cc:
	Likewise.