* 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.
* Update the comments that refer to erf[c](nan) and erf[c](+-inf)
  to use erf[c]f.