Increase s390x baseline to z15/z16

[yselkowitz]

Fedora, CentOS, and RHEL infrastructure seems generally ready for the new s390x baseline now. POWER10 is still a blocker, so let's do this in two stages.

gcc should be configured --with-arch=z15 --with-tune=z16 for s390x when 0%{?rhel} >= 11.

[yselkowitz]

https://src.fedoraproject.org/rpms/gcc/pull-request/74

[yselkowitz]

https://src.fedoraproject.org/rpms/redhat-rpm-config/pull-request/358

[yselkowitz]

https://src.fedoraproject.org/rpms/glibc/pull-request/152

[yselkowitz]

@sharkcz https://src.fedoraproject.org/rpms/libzdnn/blob/rawhide/f/libzdnn.spec#_50 overrides the default CFLAGS to enforce z14. But is z14 exactly required or just the minimum? IOW is that override necessary for RHEL 9+ which already defaults to -march=z14 -mtune=z15, or for RHEL 11 (ELN) which we are now bumping to -march=z15 -mtune=z16?

[sharkcz]

@sharkcz https://src.fedoraproject.org/rpms/libzdnn/blob/rawhide/f/libzdnn.spec#_50 overrides the default CFLAGS to enforce z14. But is z14 exactly required or just the minimum? IOW is that override necessary for RHEL 9+ which already defaults to -march=z14 -mtune=z15, or for RHEL 11 (ELN) which we are now bumping to -march=z15 -mtune=z16?

I am pretty sure the z14 arch is the minimum to ensure the right vector operations/instructions are available, but the runtime check is compiled with the distro flags. The logic in the spec file likely dates back to when z13 (or older) was the baseline. Should be fixable with %if %fedora.