Fix ldbl-128ibm tanhl inaccuracy for small arguments (bug 19349).

The ldbl-128ibm implementation of tanhl is inaccurate for small
arguments, because it returns x*(1+x) (maybe in an attempt to raise
"inexact") when x itself would be the accurate return value but
multiplying by 1+x introduces large errors.  This patch fixes it to
return x in that case (when the mathematical result is x plus a
negligible remainder on the order of x^3) to avoid those errors.

Tested for powerpc.

	[BZ #19349]
	* sysdeps/ieee754/ldbl-128ibm/s_tanhl.c (__tanhl): Return argument
	when small.