Add DIAGNOSTIC_IGNORE_STRINGOP_TRUNCATION

Add DIAGNOSTIC_IGNORE_STRINGOP_TRUNCATION for GCC 8.1 to silence
-Wstringop-truncation warning:

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85643

	* diagnostics.h (DIAGNOSTIC_STRINGIFY_1): New.
	(DIAGNOSTIC_STRINGIFY): Likewise.
	(DIAGNOSTIC_IGNORE): Replace STRINGIFY with DIAGNOSTIC_STRINGIFY.
	(DIAGNOSTIC_IGNORE_SELF_MOVE): Define empty if not defined.
	(DIAGNOSTIC_IGNORE_DEPRECATED_REGISTER): Likewise.
	(DIAGNOSTIC_IGNORE_UNUSED_FUNCTION): Likewise.
	(DIAGNOSTIC_IGNORE_SWITCH_DIFFERENT_ENUM_TYPES): Likewise.
	(DIAGNOSTIC_IGNORE_STRINGOP_TRUNCATION): New.