Fix undefined behavior: left-shifting into the sign bit.

[dim]

LGTM. Are any users of the macros affected, btw?

[kib]

In D22898#501463, @dim wrote:

Are any users of the macros affected, btw?

I do not think that anything is affected for real, otherwise we would already have reports e.g. about threading breakage due to SIGTHR == 32.
This is only a preventive measure against compiler writers trying to make our life hard.