- Use the volatile qualifier for 'tiny' to prevent compilers from erronously constant folding expressions of the form '1 - tiny'. This allows erf[f](x) to raise INEXACT.
- Use 0.5, 1, and 2, which are exactly representable in radix-2 floating point formats. This reduces diffs between s_erf[fl].c.
- While here, add a comment about efx and efx8.