shadow: Backport last change reproducibility

shadow: Backport last change reproducibility

The third field in the /etc/shadow file (sp_lstchg) contains the date of
the last password change expressed as the number of days since Jan 1,
1970.

Backport the upstream changes to honour SOURCE_DATE_EPOCH for build
reproducibility.

Signed-off-by: Alex Kiernan <alex.kiernan@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
Signed-off-by: Armin Kuster <akuster808@gmail.com>