mark all vfp registers as used when done.

To avoid assigning registers the would fit, once arguments have been on
the stack, we mark all registers as used once we do not find a free
register for the first time.
stop trying to assing vfp regs once we are done with the registers
add a testcase for the double/float issue on ARMHF
update Changelog
Merge branch 'master' of https://github.com/bivab/libffi

Conflicts:
	ChangeLog