It is reported that on a system of i386 System V ABI, GCC returns 8 for
__alignof__(double).  OTOH the ABI defines alignments of double to be 4,
and ISO/IEC 9899:2011 reads that _Alignof(double) shall return 4 on such
machine.  What we want in ruby is 4 instead of 8 there.  We cannot use
__alignof__.

Additionally, both old GCC / old clang return 8 for _Alignof(double) on
such platforms.  They are their bugs, and already fixed in recent
versions.  But we have to support older compilers for a while.  Shall
check sanity of _Alignof.