Fix ldbl-128ibm fmodl handling of equal arguments with low part zero (bug 19602).

The ldbl-128ibm implementation of fmodl has logic to detect when the
first argument has absolute value less than or equal to the second.
This logic is only correct for nonzero low parts; if the high parts
are equal and the low parts are zero, then the signs of the low parts
(which have no semantic effect on the value of the long double number)
can result in equal values being wrongly treated as unequal, and an
incorrect result being returned from fmodl.  This patch fixes this by
checking for the case of zero low parts.

Although this does show up in tests from libm-test.inc (both tests of
fmodl, and, indirectly, of remainderl / dreml), the dependence on
non-semantic zero low parts means that test shouldn't be expected to
reproduce it reliably; thus, this patch adds a standalone test that
sets up affected values using unions.

Tested for powerpc.

	[BZ #19602]
	* sysdeps/ieee754/ldbl-128ibm/e_fmodl.c (__ieee754_fmodl): Handle
	equal high parts and both low parts zero specially.
	* sysdeps/ieee754/ldbl-128ibm/test-fmodl-ldbl-128ibm.c: New test.
	* sysdeps/ieee754/ldbl-128ibm/Makefile [$(subdir) = math] (tests):
	Add test-fmodl-ldbl-128ibm.