newvers: Honour SOURCE_DATE_EPOCH as a build reproducibility measure

One reason the kernel does not build reproducibly is that it includes
a timestamp in the version string. SOURCE_DATE_EPOCH provides a standard
method to address this: it should be set to the last modification time
of the source, and build processes use the specified timestamp instead
of the "current" date and time.

This change uses SOURCE_DATE_EPOCH if it is set; how it gets set needs
to be addressed elsewhere.

Reviewed by: bapt
MFC after: 1 week
Sponsored by: The FreeBSD Foundation