PowerPC: Build fails in libffi

This failure occurs in both make and hadrian builds:

In file included from ../src/powerpc/ffi.c:33:0:
../src/powerpc/ffi_powerpc.h:61:9: error: ‘_Float128’ is not supported on this target
 typedef _Float128 float128;

There is libffi ticket 561 and a fix that did not make it into release 3.3.

The GCC manual says flag -mfloat128 is required to use _Float128.