Be compatible with the x86-32 calling convention by ensuring that the
stack is aligned to 16 bytes at the point of the call
instruction. Also ensure that vector arguments passed on the stack are
16 byte aligned.

Also, make alloca instructions respect alignment.

BUG=none
R=jvoung@chromium.org, stichnot@chromium.org

Review URL: https://codereview.chromium.org/444443002