The powerpc64 ABIs align structs passed by value, a fact ignored by
gcc for quite some time.  Since gcc now does the correct alignment,
libffi needs to follow suit.  This ought to be made selectable via
a new abi value, and the #ifdefs removed from ffi.c along with many
other #ifdefs present there and in assembly.  I'll do that with a
followup patch sometime.

This is a revised version of
https://sourceware.org/ml/libffi-discuss/2013/msg00162.html