* expr.c [CLEAR_RATIO]: New macro defining the maximum number
	of move instructions to use when clearing memory, c.f. MOVE_RATIO.
	[CLEAR_BY_PIECES]: New macro, using CLEAR_RATIO, to determine
	whether clear_by_pieces should be used to clear storage.
	(clear_storage): Use CLEAR_BY_PIECES instead of MOVE_BY_PIECES.

	* doc/tm.texi: Document these two new target macros.

From-SVN: r55429