Record TIMESTAMP in make makesum

This is intended to support development and prototyping for ports
reproducible build efforts which require some concept of a "last
updated" time. It is being committed now so that timestamp entries
will be populated "for free" as port distfiles updates happen.

Submitted by: bapt (earlier version)
Reviewed by: bapt, bdrewery
Approved by: portmgr (bdrewery)
Differential Revision: https://reviews.freebsd.org/D6031