32bit memset-sse2.S fails with uneven cache size

32bit memset-sse2.S assumes cache size is multiple of 128 bytes.  If
it isn't true, memset-sse2.S will fail.  For example, a processor can
have 24576 KB L3 cache and 20 cores. That is 2516582 byte per core. Half
of it is 1258291, which isn't helpful for vector instructions.  This
patch rounds cache sizes to multiple of 256 bytes.

(cherry picked from commit c0dde15b
but removed introduction of new variables)