Lower alloca in a way similar to x86. Subtract the stack
and align if needed, then copy that stack address to dest.
Sometimes use "bic" for the mask, sometimes use "and",
depending on what fits better.

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

Review URL: https://codereview.chromium.org/1156713003