The __x86_64__ macro identifies a CPU family, and is unfortunately not
enough to identify a concrete ABI.
The normal x86_64 ABI that is used by practical Linux distributions is
LP64 (i.e. 32-bit int, and 64-bit long and pointer), and defines
__x86_64__ and __LP64__.
x32 is a niche ILP32 ABI (i.e. 32-bit int, long and pointer) for x86_64
CPUs, which has different struct sizes due to sizeof(long) and
sizeof(void *) being smaller. It defines __x86_64__ and __ILP32__.
Signed-off-by: Simon McVittie <smcv@debian.org>