Fix ldbl-128ibm expl overflow in non-default rounding modes (bug 19078).

Fix ldbl-128ibm expl overflow in non-default rounding modes (bug 19078).

The ldbl-128ibm expl wrapper checks the argument to determine when to
call __kernel_standard_l, thereby overriding overflowing results from
__ieee754_expl that could otherwise (given appropriately patched
libgcc) be correct for the rounding mode.  This patch changes it to
check the result of __ieee754_expl instead, as other versions of this
wrapper do.

Tested for powerpc.

	[BZ #19078]
	* sysdeps/ieee754/ldbl-128ibm/w_expl.c (o_thres): Remove variable.
	(u_thres): Likewise.
	(__expl): Determine whether to call __kernel_standard_l based on
	value of result, not argument.