.. bpo: 33374
.. date: 2018-04-29-12-07-00
.. nonce: -xegL6
.. release date: 2018-04-29
.. section: Core and Builtins

Tweak the definition of PyGC_Head, so compilers do not believe it is always
16-byte aligned on x86. This prevents crashes with more aggressive
optimizations present in GCC 8.
