Fix ldbl-128ibm logbl near powers of 2 (bug 18030).

The ldbl-128ibm implementation of logbl 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, logbl
(0x1.ffffffffffffffp1L) returns 2 but should return 1.  (This is
similar to (fixed) bug 16740 for frexpl, and (fixed) bug 18029 for
ilogbl.)  This patch adds checks for that case.

Tested for powerpc.

	[BZ #18030]
	* sysdeps/ieee754/ldbl-128ibm/s_logbl.c (__logbl): Adjust exponent
	of power of 2 down when low part has opposite sign.
	* math/libm-test.inc (logb_test_data): Add more tests.

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