Build timestamps prevent reproducible builds [0].

[0] https://reproducible-builds.org/docs/timestamps/

Signed-off-by: Alexander Couzens <lynxis@fe80.eu>

This fixes an alignment issue on some platforms, which renders
some tools, e.g. plcID, unusable.

While at, switch to xz compression for the tarball, drop an already
mainlined patch and remove a meanswhile absolete compiler switch.

Signed-off-by: Michael Heimpold <mhei@heimpold.de>

- Include `poll.h` at the top of `ether/readpacket.c` to avoid nested
   declaration errors caused by fortify source headers
 - Add missing `sys/select.h` include to `serial/serial.c`

Signed-off-by: Jo-Philipp Wich <jow@openwrt.org>