libelftc: add elftc_timestamp to handle SOURCE_DATE_EPOCH

[emaste]

Summary

SOURCE_DATE_EPOCH[1] specifies a timestamp (number of seconds since the UNIX epoch) to be used as a timestamp in place of the "current" date and time, in order to facilitate reproducible builds.

[1] https://reproducible-builds.org/specs/source-date-epoch/

(Note, this diff is against the upstream elftoolchain repo, and would make it into FreeBSD in a future vendor import.)

[kaiw]

The manual page is missing a ".El" between .Xr strtoll 3 . and .Sh SEE ALSO
Otherwise the patch looks quite good! Thanks!

[kaiw]

accept revision (only a minor change in the manual page needed, as stated in the comment)

[emaste]

Committed in http://sourceforge.net/p/elftoolchain/code/3489 (with .El fix), thank you for reviewing!