re PR libstdc++/50880 (__complex_acosh() picks wrong complex branch)

2011-11-02  Richard B. Kreckel  <kreckel@ginac.de>
	    Paolo Carlini  <paolo.carlini@oracle.com>

	PR libstdc++/50880
	* include/std/complex (__complex_acosh): Fix in a better way,
	use Kahan's formula.
	* include/tr1/complex (__complex_acosh): Likewise.

From-SVN: r180788