Fix ldbl-128ibm truncl for non-default rounding modes (bug 19593).

Fix ldbl-128ibm truncl for non-default rounding modes (bug 19593).

The ldbl-128ibm implementation of truncl is only correct in
round-to-nearest mode (in other modes, there are incorrect results and
overflow exceptions in some cases).  It is also unnecessarily
complicated, rounding both high and low parts to the nearest integer
and then adjusting for the semantics of trunc, when it seems more
natural to take the truncation of the high part (__trunc optimized
inline versions can be used), and the floor or ceiling of the low part
(depending on the sign of the high part) if the high part is an
integer, as was done for floorl and ceill.  This patch makes it use
that simpler approach.

Tested for powerpc.

	[BZ #19593]
	* sysdeps/ieee754/ldbl-128ibm/s_truncl.c (__truncl): Use __trunc
	on high part and __floor or __ceil on low part then use
	ldbl_canonicalize_int if needed.