Make compile_time_assert compatible with x32 (Closes: #1063738)

The x32 build was failing on:

compile_time_assert(sizeof(MYSQL) == 77*sizeof(void*)+656);

Reported upstream at https://jira.mariadb.org/browse/MDEV-34195. This is a regression from upstream commits c432c9ef and 06a884a5.

Fix is based on advice from https://lists.debian.org/debian-amd64/2024/07/msg00003.html which is also aligned with https://wiki.debian.org/X32Port.