Fixed the sign of the result in some overflow and underflow cases (ones

Fixed the sign of the result in some overflow and underflow cases (ones
where the exponent is an odd integer and the base is negative).

Obtained from:	fdlibm-5.3

Sun finally released a new version of fdlibm just a coupe of weeks
ago.  It only fixes 3 bugs (this one, another one in pow() that we
already have (rev.1.9), and one in tan().  I've learned too much about
powf() lately, so this fix was easy to merge.  The patch is not verbatim,
because our base version has many differences for portability and I
didn't like global renaming of an unrelated variable to keep it separate
from the sign variable.  This patch uses a new variable named sn for
the sign.