elfcopy: support SOURCE_DATE_EPOCH

[emaste]

Summary

1. libelftc: add elftc_timestamp to handle SOURCE_DATE_EPOCH

SOURCE_DATE_EPOCH specifies a timestamp (number of seconds since the UNIX epoch) to be used as the embedded timestamp in place of the "current" date and time, in order to facilitate reproducible builds.

SOURCE_DATE_EPOCH spec: https://reproducible-builds.org/specs/source-date-epoch/

2. elfcopy: use elftc_timestamp, to support SOURCE_DATE_EPOCH

Ref: https://twitter.com/delphij/status/741047758992216064

[emaste]

For clarity (as this review will probably be referenced elsewhere), I'm proposing this change initially for upstream ELF Tool Chain, with the intent to bring it into FreeBSD via a later vendor branch update.

[emaste]

See D7631 and D7632 instead