libstdc++: Fix failing test in debug mode

libstdc++: Fix failing test in debug mode

This fixes a failure due to a (correct) warning seen when testing with
-D_GLIBCXX_USE_CXX11_ABI=0 -D_GLIBCXX_ASSERTIONS:

include/bits/char_traits.h:365: warning: 'void* __builtin_memcpy(void*, const void*, long unsigned int)'
specified bound 18446744073709551615 exceeds maximum object size 9223372036854775807 [-Wstringop-overflow=]

FAIL: 21_strings/basic_string/cons/char/1.cc (test for excess errors)

	* testsuite/21_strings/basic_string/cons/char/1.cc: Disable
	-Wstringop-overflow warnings.