Add a separate image for gcc 11

[am11]

GNU compiler toolchain is used in runtime for native build validation. The LLVM toolchain is default one to build the shipping product.

Currently we are using gcc9 from centos-7 image, which we install from YUM packages when building the prereq image for two purposes:

1. to compile custom llvm toolchain
2. to use it in for the said validation leg in runtime repo

Therefore, it is non-trivial to upgrade gcc independently, as first we need to wait fro devtoolset to pick up the GNU version, then we need to make sure our custom llvm toolchain's build is intact.

This PR adds gcc 11 leg based on official library layer (https://hub.docker.com/_/gcc), which uses Debian 11 (buillseye) for gcc 11. In future, we can add src/debian/{version}/gcc/Dockerfile when needed, without affecting anything else.

[am11]

cc @janvorli, @hoyosjs, @wfurt
I have tested it locally, will send a PR in runtime to switch gcc leg to this container once we have the new image produced.

[am11]

Failures are in unrelated legs (Ubuntu certificates are expired)

[am11]

Rebased with main.
Ubuntu error went away, since it was fixed by #505.
Speaking of moving targets, between yesterday and today, Fedora 34 regressed due to pip marked as optional dependency... opened #506 with the fix.

[am11]

All green. Lets snapshot it now before it regresses. 😅

@hoyosjs, @janvorli, this will potentially fix all 2565 warnings currently we are hitting in gcc leg, enable us to unsuppress -Wall in the JIT and hopefully will close dotnet/runtime#33541. Those warnings seem to be fixed in gcc 11.

[hoyosjs]

LGTM if the runtime properly builds on this container. Thanks @am11!

[am11]

Thanks! @mthalman, if it LGTYT, please merge.