stand: Use SOURCE_DATE_EPOCH in newvers.sh

[cperciva]

Summary

The newvers.sh script is used to generate vers.c for 10 programs (at
latest count on amd64): 4 EFI loaders, 3 x86 loaders, two userboots,
and one kboot. When building without MK_REPRODUCIBLE_BUILD this
resulted in 10 different timestamps being compiled into various files.

If SOURCE_DATE_EPOCH is set, use that value rather than the current
date and time.

Sponsored by: Amazon

[cperciva]

Context which probably doesn't need to be in the commit message: I'd like to make builds on HEAD *reproducible* even if they're not *deterministic*, i.e. given a build I want to be able to extract environment settings which will allow the same build to be reproduced. This will be easier if I can SOURCE_DATE_EPOCH?= a single SOURCE_DATE_EPOCH value and have it apply to all of the loader binaries.

[emaste]

OK. My preference would be to remove the date altogether, regardless of build settings, but this is an incremental step at least.