powerpc: Fix logbl on power7 [BZ# 21280]

 1. Fix the results for negative subnormals by ignoring the signal when
    normalizing the value.
 2. Fix the output when the high part is a power of 2 and the low part
    is a nonzero number with opposite sign.  This fix is based on commit
    380bd0fd.

After applying this patch, logbl() tests pass cleanly on POWER >= 7.

Tested on powerpc, powerpc64 and powerpc64le

	[BZ #21280]
	* sysdeps/powerpc/power7/fpu/s_logbl.c (__logbl): Ignore the
	signal of subnormals and adjust the exponent of power of 2 down
	when low part has opposite sign.