Avoid some left-shifts of negative constants

[Paul Eggert]

On 2/6/19 1:37 PM, Joseph Myers wrote:
> One group of warnings seen with -Wextra is "left shift of negative
> value [-Wshift-negative-value]".

This group of warnings is make-work, right? Glibc assumes GCC, and in 
GCC the left shift of a negative value is well-defined unless it 
overflows (which can't happen in this case). So I suggest that we add 
-Wno-shift-negative-value to CFLAGS instead of worrying about pacifying 
the compiler by munging the source code.