Avoid unnecessarily dirtying pages in SemiSpace collector.

Added a special version of memcpy which has handling to avoid dirty
pages for objects which are larger than one page. No non noise GC
performance regression measured on ritzperf benchmark. Seems to save
4k to 148k memory per app on AOSP mako.

Bug: 10802951
Change-Id: I5976c83643e06beebf5c86a65c584bf7c1ab1e2e