libffi: fix powerpc build breakage

Happens with the latest release when there are no FP registers.
Fixes:
http://autobuild.buildroot.net/results/bc43261d3ddc9d4c320522563249f4a0695a35a4/



Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>