libm: Avoid left-shifting negative integers.

It is undefined behaviour.

Taken-from: OpenBSD

Import OpenBSD's libm (trunk, 4 July 2015) to a new vendor branch

With the exception of x86_64 (as amd64), all arch-specific files have
been filtered out. A new directory called "include" has been

Import OpenBSD's libm (trunk, 4 July 2015) to a new vendor branch

With the exception of x86_64 (as amd64), all arch-specific files have
been filtered out. A new directory called "include" has been added
to capture global and arch-specific headers for installation. Other
than that directory and those exclusions, this is an unaltered copy
of OpenBSD's libm as it existed on 4th of July, 2015.

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