gcc-10 update for bullseye/bullseye-backports

[Matthias Klose <doko@debian.org>]

Hi,

I never got feedback on the binutils/GCC plans outlined last July
https://lists.debian.org/debian-release/2020/07/msg00008.html

At this point, I don't think another gcc-10 upload is needed, as people can work
around existing issues by using gcc-9, which still is used as a build
dependency.  I will prepare a gcc-10 package for bullseye-backports, based on
the upstream 10.3.0 release.

The upgrade issue reported as #972936 is not confirmed, and missing feedback if
the suggested solution is working.

To check the current status of gcc-10 as found in experimental, I built a
package for unstable (including the suggested fix for #972936) at

  deb [trusted=yes] http://people.debian.org/~doko/gcc/gcc10 ./

and asked Lucas Nussbaum for a comparative test rebuild.  Results can be found at

https://people.debian.org/~doko/logs/20210228/diff.vanilla-gcc10

Progressions:
$ awk '/Failed OK/ {print $1}' diff.vanilla-gcc10
gammaray
mkvtoolnix
nheko
odb

Regressions:
$ awk '/OK Failed/ {print $1}' diff.vanilla-gcc10
gsequencer

The latter filed as #983868, having both a work around and an upstream fix.

Matthias