Use fixed mtime for tar packages

This allows to have fixed checksums for packages built
at different times.

We use the author date of the mgwport/msysport file in seconds since
epoch and use 2000-01-01 as fallback.

Signed-off-by: Thomas Braun <thomas.braun@byte-physics.de>
Add -n -9 as gzip flags for producing deterministic archives

This is in line what debian uses [1].

From gzip's help output:
-n, --no-name do not save or restore the original name and time stamp
-9, --best compress better

[1]: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=647522

Signed-off-by: Thomas Braun <thomas.braun@byte-physics.de>
Use --enable-deterministic-archives flag for strip from binutils in its mingw flavour

Signed-off-by: Thomas Braun <thomas.braun@byte-physics.de>