Among other things, this gets us GCC 10 (was 6).

Requires some changes to third party components we use:

* Install apitrace (& waffle) from Debian; was hitting issues with the
  local build, and it's the same version 9.0 anyway.
* Update Fossilize to a newer commit which builds with GCC 10.
* apt.llvm.org repositories are no longer needed.
* Use an SPIRV-LLVM-Translator commit which builds with LLVM 11.0.1.
* Install XCB packages from Debian, 1.13 fails to build with Python 3.9.
* Install wayland-protocols from Debian, 1.12 is too old for
  libgtk-3-dev in bullseye.

LLVM 7/8 packages are no longer available.

Also adapt expected test results to Xvfb now exposing multi-samle
GLXFBConfigs.

v2:
* Install clang instead of clang-11.

Closes: mesa/mesa#3124
Reviewed-by: Eric Anholt <eric@anholt.net> # v1
Part-of: <mesa/mesa!9833>