* libm/common/s_ilogb.c: Adjust documentation to reflect new returns
(see next items) and that ilogb() and ilogbf() are C99/POSIX.
* libm/common/s_ilogb.c (ilogb): Use C99/POSIX FP_ILOGB0 and
FP_ILOGBNAN returns for 0 and NAN arguments, respectively, instead of
the prior -INT_MAX and INT_MAX. The FP_ILOGBx defines in math.h
presently match the prior hard-codes, so no functional change is
actually introduced. (Not at this time, nor planned, but the values
are permitted to be different, so subsequent edits of math.h could
possibly cause a functional change.)
* libm/common/sf_ilogb.c (ilogbf): Ditto.
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