Fix sign of input to bsloww1 (BZ #16623)

In 84ba214c, I removed some redundant sign computations and in the
process, I incorrectly got rid of a temporary variable, thus passing
the absolute value of the input to bsloww1.  This caused #16623.

This fix undoes the incorrect change.

(cherry picked from commit ffe768a9)

https://bugs.gentoo.org/509494
https://sourceware.org/bugzilla/show_bug.cgi?id=16623