Update gcc CI to use gcc-11 container

[am11]

With gcc9 leg, we had 2500+ warnings in JIT due to a bit field width issue: #33541.

Now that we have a dedicated gcc container, it is easier to upgrade gcc toolchain independent from llvm toolchain.

This PR uses the latest gcc 11 prereq image. The bitfield width warnings have been fixed in this version, therefore I have enabled Werror in the JIT for gcc version >= 11.

Fixes #33541

[dotnet-issue-labeler]

I couldn't figure out the best area label to add to this PR. If you have write-permissions please help me learn by adding exactly one area label.

[am11]

cc @janvorli, @BruceForstall

[am11]

This condition was missed out previously, and was failing the libs.native subset build with gcc (which we currently don't exercise in the CI and maybe we can; since it takes 30-60 seconds max)

[am11]

gcc leg has picked up the newer version:

Invoking "/__w/1/s/eng/native/gen-buildsys.sh" "/__w/1/s/src/coreclr" "/__w/1/s/artifacts/obj/coreclr/Linux.x64.Checked" x64 gcc "" "" Checked ""  -DCLR_CMAKE_PGO_INSTRUMENT=0 -DCLR_CMAKE_OPTDATA_PATH=/__w/1/s/.packages/optimization.linux-x64.pgo.coreclr/1.0.0-prerelease.21460.6 -DCLR_CMAKE_PGO_OPTIMIZE=1 -DFEATURE_DISTRO_AGNOSTIC_SSL=1 
/__w/1/s/artifacts/obj/coreclr/Linux.x64.Checked /__w/1/s
Not searching for unused variables given on the command line.
-- The C compiler identification is GNU 11.2.0
-- The CXX compiler identification is GNU 11.2.0
...

no compiler warnings. There is one preexisting assembler warning which remained, however:

[ 47%] Building CXX object jit/CMakeFiles/clrjit_unix_x64_x64.dir/bitset.cpp.o
/tmp/cchOQSqP.s: Assembler messages:
/tmp/cchOQSqP.s:3245: Warning: ignoring changed section attributes for .rodata
[ 47%] Building CXX object ildasm/exe/CMakeFiles/ildasm.dir/__/dis.cpp.o

I haven't looked too deep but I think it is not related to GSCookie, or it is benign. The compiled binaries do have all the exported symbols in correct segments which we typically find when compiled with clang (like the ones which SOS looks for).

[janvorli]

LGTM, thank you!