mac: Update executable padding

x86_64 (offset + size) / alignment = (286256 + 16384) / 2^14 = 18.4716.
We want 17.5, so change padding by 2^14 * (17.5 - 18.4716) = -15918.6944
bytes = -15.5456 KiB, rounded to the nearest 4 KiB is nearly -16 KiB.

Also, modified the comment slightly to be written as an example, because
it is obnoxious to always update all the numbers in the comment to match
the padding (which changes over time).

Fixed: 410680385
Change-Id: Ie3f8ba4832e83f700c127a00a723eb6790591403
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6458857
Auto-Submit: Joshua Pawlicki <waffles@chromium.org>
Commit-Queue: Leonard Grey <lgrey@chromium.org>
Reviewed-by: Leonard Grey <lgrey@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1447102}