MDEV-34195 tests: avoid LP64 MYSQL size assert on x32

MDEV-34195 tests: avoid LP64 MYSQL size assert on x32

x32 defines __x86_64__ but uses the ILP32 ABI, so the LP64
sizeof(MYSQL) == 1272 compile-time assert is selected incorrectly
and breaks the build.

Guard the x86_64 check with !defined(__ILP32__) so the LP64 size assert
only applies to real amd64 LP64 builds, while leaving the existing
Windows amd64 and i386 checks unchanged.

(cherry-picked from 936f04c4f9bd5d66014942bd27e1996b2e740626)

Merged on 10.11, and will be included in next release (10.11.19+, 11.8.9+).

Forwarded: https://github.com/MariaDB/server/pull/4962

Gbp-Pq: Name MDEV-34195-tests-avoid-LP64-MYSQL-size-assert-on-x32.patch