Work-around libffi's FP ABI detection.

Work-around libffi's FP ABI detection.

On iOS, we must use the AAPCS floating point return value calling
conventions. libffi's ARM implementation will only use these conventions
if __SOFTFP__ is defined, which is not the case when GCC's
-mfloat-abi defaults to 'softfp' instead of 'soft'. To work around this
we manually define __SOFTFP__ for Apple platforms in the ARM-specific
sysv.S.

See also:
  http://developer.apple.com/library/ios/#documentation/Xcode/Conceptual/iPhoneOSABIReference/Introduction/Introduction.html
  http://infocenter.arm.com/help/topic/com.arm.doc.ihi0042d/IHI0042D_aapcs.pdf