RFA: Fix signbit() for 16-bit targets

[Craig Howland]

On 05/12/2015 06:13 PM, DJ Delorie wrote:
>> -  return (w & 0x80000000);
>> +  return (w & 0x80000000) != 0;
> Also 0x80000000 -> 0x80000000UL ?
Given that uint32_t is being used, UINT32_C(0x80000000) would seem most 
appropriate, although this might mean stdint.h would have to be added.
Craig