- 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/
- elfcopy: use elftc_timestamp, to support SOURCE_DATE_EPOCH