[release/3.1] Fix building with clang 13

[omajid]

Clang now enables cast-function-type warning which results in build
errors that look like this:

/corefx/src/Native/Unix/System.Native/pal_process.c(382,76): error G39B05358: cast from 'void (*)(int, siginfo_t *, void *)' to 'void (*)(int)' converts to incompatible function type [-Werror,-Wcast-function-type] [/corefx/src/Native/build-native.proj]
void (*oldhandler)(int) = (sa_old.sa_flags & SA_SIGINFO) ? (void (*)(int))sa_old.sa_sigaction : sa_old.sa_handler;
                                                           ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Disable this warning using -Wno-cast-function-type warning. That seems
lower-risk than any functional code change.

Clang 13 also enables reserved-identifier warning which results in build errors
like this:

/corefx/src/Native/Unix/System.Native/pal_process.c(819,17): error GA61F72A6: identifier '__cpu' is reserved because it starts with '__' [-Werror,-Wreserved-identifier] [/corefx/src/Native/build-native.proj]
                  if (CPU_ISSET(cpu, &set))
                      ^
/usr/include/sched.h:94:34: note: expanded from macro 'CPU_ISSET'
# define CPU_ISSET(cpu, cpusetp) __CPU_ISSET_S (cpu, sizeof (cpu_set_t), \
                                 ^

This really seems like a bug in clang that it's complaiing about code in
system libraries that we are using via wrapper-macros. But there are
other instances of this warning that are speciifc to our code too, such
as __padding in
src/Native/Unix/System.Native/pal_interfaceaddresses.h. Disable this
warning too.

Tested with Clang 13 rc1

[safern]

@omajid what's the status of this? Is this still needed?

[omajid]

Yes, still needed: we would like to build .NET Core 3.1 on recent versions of Fedora (and even Alpine, see #43130) that fail because of warnings.

(The alternative is each Linux distro carrying the patches locally, but that seems unnecessary for well understood changes like disabling compiler warnings).

[safern]

Ok, would you mind rebasing on top of the target branch and pushing that?

[omajid]

Yup, just wanted to find some time to see if this improves things on alpine too.

[omajid]

@safern I have rebased it and re-tested it.

[omajid]

Test failures are in libraries, which look unrelated to my changes?

[danmoseley]

The failures are unrelated but would be fixed by grabbing these lines. Could go in this PR or another.
https://github.com/dotnet/runtime/pull/62559/files#diff-18bd39fa893065cf18d4932a2a06e9796381e5548a5a809d3fd53c83b3bdb14dR131-R144

[safern]

Branch is closed as of now. But we can merge this when is open.

Thanks, @danmoseley for the pointer. If @omajid has time to port that it can be done here, if not I'll do it in a separate PR.

[carlossanlop]

@omajid the branch is open again. Can you please port the suggested lines mentioned above to get rid of the failure?

[omajid]

The test fixes were ported separately by #43137.

I have rebased this PR on top of that.

[carlossanlop]

@hoyosjs @jkotas need to consult with you: do you have any objections on taking this 3.1 change? And do cmake changes need to go through Tactics, or can we just consider them infra [tell-mode]?

[jkotas]

Have you looked at the CI failures and determined that they are unrelated?

This is same type of change as #42900. It is fine to take it. It should follow the same process as #42900 .

[hoyosjs]

/azp run corefx-ci

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[hoyosjs]

The changes themselves make sense and are the least risky for servicing. @danmoseley can this be servicing-consider?

[danmoseley]

Is it worth it? It is out of support in 4 months.

[carlossanlop]

Closing since 3.1 is going out of support.