Enable compiler checks for printf-like SBuf methods (#1270)

Enable compiler checks for printf-like SBuf methods (#1270)

After commit 0f17e25, these two are the only known printf()-like
functions not already covered by these compiler checks. Some helper
debugging hacks do not have PRINTF_FORMAT_ARG2 or similar attributes,
but they are covered because their `__GNUC__` variants use standard
fprintf().