Bumping the baseline on ppc64el in jammy to POWER9

[Matthias Klose]

This weekend, we will bump the baseline for the ppc64el architecture to POWER9.
A test compiler (gcc-11) for jammy can be found at

  https://launchpad.net/~ubuntu-toolchain-r/+archive/ubuntu/p9

We will not do an explicit test rebuild for that change, but do the first test
rebuild during the Xmas/New Year breaks with this p9 default.  If bugs are found
with these defaults,

 - fix the bug if possible

 - file a bug report with the tag "power9"

 - if a fix is urgently needed, file the bug report as
   above, and then upload the package with a workaround
   defaulting to the previous baseline:

     -march=power8 -mtune=power9

Matthias