x32: mariadb: if defined __ILP32__ and __x86_64__ ?

Hi!

MariaDB has this piece of code that is failing on x32 as reported in
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1063738

tests/mysql_client_fw.c
@@ -1442

#if defined __x86_64__
  compile_time_assert(sizeof(MYSQL) == 1272);
#elif defined __i386__
  compile_time_assert(sizeof(MYSQL) == 964);
#endif

How should I fix this for x32? What is the preprocessor if clause and
can I safely assume the size is 964?

Previous x32 builds passed before upstream introduced this change. References:
- https://wiki.debian.org/X32Port
- https://buildd.debian.org/status/logs.php?pkg=mariadb&arch=x32
- https://salsa.debian.org/mariadb-team/mariadb-server