Fix ldbl-128ibm ilogbl near powers of 2 (bug 18029).

The ldbl-128ibm implementation of ilogbl produces incorrect results
when the high part of the argument is a power of 2 and the low part a
nonzero number with the opposite sign (and so the returned exponent
should be 1 less than that of the high part).  For example, ilogbl
(0x1.ffffffffffffffp1L) returns 2 but should return 1.  (This is
similar to (fixed) bug 16740 for frexpl, and bug 18030 for logbl.)
This patch adds checks for that case.

Tested for powerpc.

	[BZ #18029]
	* sysdeps/ieee754/ldbl-128ibm/e_ilogbl.c (__ieee754_ilogbl):
	Adjust exponent of power of 2 down when low part has opposite
	sign.
	* math/libm-test.inc (ilogb_test_data): Add more tests.