On MIPS we can get better code by having can_store_by_pieces differentiate 
between the cases where it's used for memset operations, and those where it's 
used to copy a string constant.  This patch introduces new SET_RATIO and 
SET_BY_PIECES_P macros, with appropriate defaults to preserve the existing 
behavior.  I checked other targets and made the ones that override the default 
STORE_BY_PIECES_P clone the same definition for SET_BY_PIECES_P.

I used CSiBE to run some code size measurements with -Os.  Together, these bits 
add up to about 0.5% total space savings for both MIPS32R2 and MIPS64.  With 
-mips16 there was just a tiny improvement.

Look OK?  I haven't yet tested on other platforms besides MIPS but will do a 
bootstrap on i686 before checking it in, if there are no objections to the patch 
otherwise.

-Sandra