s390x: Fedora ELN switched to z13 baseline (in redhat-rpm-config)

[Florian Weimer]

We established a while back the the Fedora infrastructure is capable of
running z13 binaries for the IBM Z (s390x) architecture, so I do not
expect any issues.

The rationale for this change is that this aligns with the toolchain
default in Red Hat Enterprise Linux 8.

The default built into gcc will be updated in a subsequent GCC update.
It is usually overridden by the redhat-rpm-config settings anyway (if
build flags injection is working at all).

Thanks,
Florian