Lower stacksave/restore.
Lower ctlz, cttz, bswap, and popcount. Popcount is just
done with a helper call. Ctz can use the clz instruction
after reversing the bits.

We can only crosstest stacksave/restore for now which
happens to be written in C for the C99 VLAs. The CXX
crosstests I can't seem to compile with the arm-cross-g++
(missing headers), so I will check that later after
resolving the cross compilation issue.

BUG= https://code.google.com/p/nativeclient/issues/detail?id=4076
R=jpp@chromium.org

Review URL: https://codereview.chromium.org/1222943003 .