sv.c: Always #define COW threshold constants

They are also used in determining whether an operator target (PADTMP)
should be swiped.

If COW is not enabled, then the #if SV_COW_THRESHOLD ends up treating
the threshold as 0, which means swiping almost always happens for tar-
gets, resulting in an increase in malloc calls.  So non-COW builds end
up slower than they need to be.  This also caused bug #123029 to occur
more often under non-COW builds.