fix: Revert "Makefile: Enable -Wimplicit-fallthrough for Clang" (v5.15)

fix: Revert "Makefile: Enable -Wimplicit-fallthrough for Clang" (v5.15)

Starting with v5.15, "-Wimplicit-fallthrough=5" was added to the build
flags which requires the use of "__attribute__((__fallthrough__))" to
annotate fallthrough case statements.

See upstream commit by the man himself:

  commit d936eb23874433caa3e3d841cfa16f5434b85dcf
  Author: Linus Torvalds <torvalds@linux-foundation.org>
  Date:   Thu Jul 15 18:05:31 2021 -0700

    Revert "Makefile: Enable -Wimplicit-fallthrough for Clang"

    This reverts commit b7eb335e26a9c7f258c96b3962c283c379d3ede0.

    It turns out that the problem with the clang -Wimplicit-fallthrough
    warning is not about the kernel source code, but about clang itself, and
    that the warning is unusable until clang fixes its broken ways.

    In particular, when you enable this warning for clang, you not only get
    warnings about implicit fallthroughs.  You also get this:

       warning: fallthrough annotation in unreachable code [-Wimplicit-fallthrough]

    which is completely broken becasue it

     (a) doesn't even tell you where the problem is (seriously: no line
         numbers, no filename, no nothing).

     (b) is fundamentally broken anyway, because there are perfectly valid
         reasons to have a fallthrough statement even if it turns out that
         it can perhaps not be reached.

    In the kernel, an example of that second case is code in the scheduler:

                    switch (state) {
                    case cpuset:
                            if (IS_ENABLED(CONFIG_CPUSETS)) {
                                    cpuset_cpus_allowed_fallback(p);
                                    state = possible;
                                    break;
                            }
                            fallthrough;
                    case possible:

    where if CONFIG_CPUSETS is enabled you actually never hit the
    fallthrough case at all.  But that in no way makes the fallthrough
    wrong.

    So the warning is completely broken, and enabling it for clang is a very
    bad idea.

    In the meantime, we can keep the gcc option enabled, and make the gcc
    build use

        -Wimplicit-fallthrough=5

    which means that we will at least continue to require a proper
    fallthrough statement, and that gcc won't silently accept the magic
    comment versions. Because gcc does this all correctly, and while the odd
    "=5" part is kind of obscure, it's documented in [1]:

      "-Wimplicit-fallthrough=5 doesn’t recognize any comments as
       fallthrough comments, only attributes disable the warning"

    so if clang ever fixes its bad behavior we can try enabling it there again.

Change-Id: Iea69849592fb69ac04fb9bb28efcd6b8dce8ba88
Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>