sim: sprinkle some ATTRIBUTE_PRINTF

sim: sprinkle some ATTRIBUTE_PRINTF

Add some ATTRIBUTE_PRINTF attributes to functions that take a format
string, to fix a few -Wformat-nonliteral warnings.  Use the
ATTRIBUTE_PRINTF macro like we use in GDB, instead of spelling out
__attribute__((format...)).  Use ATTRIBUTE_NULL_PRINTF at one place,
because callers expect to be able to pass NULL.

sim/common/ChangeLog:

	* callback.c (os_printf_filtered, os_vprintf_filtered,
	os_evprintf_filtered, os_error): Use ATTRIBUTE_PRINTF.
	* sim-engine.h (sim_engine_abort, sim_engine_vabort): Likewise.
	* sim-events.h (sim_events_schedule_tracef,
	sim_events_schedule_vtracef): Use ATTRIBUTE_NULL_PRINTF.

Change-Id: Icd206f7b2c325e8b144f72eb129fb2a6b5af2fa3