package/zlib: update architectures for zlib-ng

- powerpc is supported since
  https://github.com/zlib-ng/zlib-ng/commit/b81f4ee96dcbdf1db34b00727b6f1829a2ba1edb
- s390x is supported since
  https://github.com/zlib-ng/zlib-ng/commit/8e30d1173699412c11880db10de2bc10893a2448



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>