Add VPUSH/VPOP instructions to the ARM32 integrated assembler.

Also fixes the corresponding emit methods for vpush and vpop to match
constraint that the maximum number of consecutive registers that can
be pushed/popped is 16.

BUG= https://bugs.chromium.org/p/nativeclient/issues/detail?id=4334
R=jpp@chromium.org, stichnot@chromium.org

Review URL: https://codereview.chromium.org/1532233002 .
6 files changed