Return proper value for ctanh with an infinite real.

Infinite real returns a signed zero imaginary, with the sign matching
what the sign of sin (2 * __imag__ z) would be.

mingw-w64-crt/ChangeLog:

2010-07-05  Doug Semler  <dougsemler@gmail.com>

	* complex/complex_internal.h: Add defines for ABI epsilon values.
	* complex/ctanh.def.h (ctanh): Return signed zero matching the
	  sign of sin(2 * __imag__ z).

git-svn-id: svn+ssh://svn.code.sf.net/p/mingw-w64/code/trunk@2778 4407c894-4637-0410-b4f5-ada5f102cad1