PPC/S390 has been droped 32/31-bit support.
So adjust the padding size for 64-bit only.

Change-Id: I3533ef4a90bee0b1e6f49aeb61498ce3054e85e7
Reviewed-on: https://chromium-review.googlesource.com/c/1397866Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Junliang Yan <jyan@ca.ibm.com>
Cr-Commit-Position: refs/heads/master@{#58598}