Bump minimum supported s390x arch-level to z10

This has been done to support OpenSSL 3.5 that uses the `cijne` opcode.

This is still a very conservative arch-level.
Gentoo also made this change when updating to OpenSSL 3.5, and to quote
the news item from Andreas K. Hüttel:
> The z10 Enterprise Class (2097 series) was introduced in February 2008,
> which essentially means everyone except hardware archaeologists should be fine.

Author: Edward-Knight

cpython-unix/targets.yml

@@ -552,6 +552,9 @@ s390x-unknown-linux-gnu:
   host_cxx: /usr/bin/x86_64-linux-gnu-g++
   target_cc: /usr/bin/s390x-linux-gnu-gcc
   target_cxx: /usr/bin/s390x-linux-gnu-g++
+  target_cflags:
+    # set the minimum compatibility level to z10 (released 2008)
+    - '-march=z10'
   needs:
     - autoconf
     - bdb