Architecture baseline for Forky

Hi

We never did a real discussion about architecture baselines before, but I think
we should do that.  We also don't have any guidelines what we as Debian want to
actually support.  But given that we are a general purpose distribution, we
have to find a balance.

As a general guidance I would like to aim for a ten to 15 years support range
at release time.  The cutoff in respect to the expected 2027 release date of
Forky would therefor be 2012 to 2017.  More time is given for widely used
architectures, less for more specialized ones.

Bastian

# Current architecture baseline

Only three architectures set an explicit baseline in gcc-12 (Bookworm) and
gcc-15 (Forky):
* armhf: armv7-a+fp
* i386: i686
* s390: z196

All other architectures use the default values.

# Proposal for architecture baseline

## amd64 (and i386)

* x86-64-v2: Supported since around 2008[^x86]. Used in RedHat 9[^redhat9x86].
* x86-64-v3: Supported since around 2013-2015[^x86]. Used in RedHat
  10[^redhat10].

I propose to use the x86-64-v2 baseline in Forky.  It gives us more then the 15
years and everyone else already switched to it or even a newer variant.
Factoring in a possible LTS for Trixie, this means that x86-64-v1 will stop
being supported by Debian in 2030.

## ppc64el

* POWER8: Launched in 2014[^power8].
* POWER9: Launched in 2017[^power9]. Used in RedHat 9[^redhat9] and
  10[^redhat10].

I propose to use the POWER9 baseline in Forky.  The Debian infrastructure seems
to be comprised of all POWER9s.  This means POWER8 will stop being supported in
2028.

## s390x

* z14: Launched in 2017[^z14]. Used by RedHat 10[^redhat10].
* z15: Launched in 2019[^z15]. Used by next Ubuntu.

I propose to use the z15 baseline in Forky.  Yes, this is less then ten years,
but s390 is such a niche architecture and all we do is best (often not even
this) effort support.  The Debian infrastructure is documented to be at least
z15, all accesible system are actually z16.  This means that z14 and older will
stop being support in 2028.

# How to see supported baseline

The dynamic linker of glibc can show what microarchiture level it considers
supported on the current CPU.  The "supported" marker is relevant in this case.

## amd64
```
% /usr/lib64/ld-linux-x86-64.so.2 --help | sed -n '/^Subdirectories of glibc-hwcaps/,$p'
Subdirectories of glibc-hwcaps directories, in priority order:
  x86-64-v4
  x86-64-v3 (supported, searched)
  x86-64-v2 (supported, searched)
```

## ppc64el
```
% /usr/lib64/ld64.so.2 --help | sed -n '/^Subdirectories of glibc-hwcaps/,$p'          
Subdirectories of glibc-hwcaps directories, in priority order:
  power10
  power9 (supported, searched)
```

## s390x
```
% /usr/lib/ld64.so.1 --help | sed -n '/^Subdirectories of glibc-hwcaps/,/^  z15/p'           
Subdirectories of glibc-hwcaps directories, in priority order:
  z16 (supported, searched)
  z15 (supported, searched)
```

[^power8]: https://en.wikipedia.org/wiki/POWER8
[^power9]: https://en.wikipedia.org/wiki/POWER9
[^x86]: https://en.wikipedia.org/wiki/X86-64#Microarchitecture_levels
[^z14]: https://en.wikipedia.org/wiki/IBM_z14_(microprocessor)
[^z15]: https://en.wikipedia.org/wiki/IBM_z15_(microprocessor)
[^redhat9]: https://docs.redhat.com/en/documentation/red_hat_enterprise_linux/9/html/9.0_release_notes/architectures
[^redhat9x86]: https://developers.redhat.com/blog/2021/01/05/building-red-hat-enterprise-linux-9-for-the-x86-64-v2-microarchitecture-level
[^redhat10]: https://docs.redhat.com/en/documentation/red_hat_enterprise_linux/10/html/considerations_in_adopting_rhel_10/architectures