objtool: Use 'the fallthrough' pseudo-keyword

Replace the existing /* fallthrough */ comments with the
new 'fallthrough' pseudo-keyword macro:

  https://www.kernel.org/doc/html/v5.7/process/deprecated.html?highlight=fallthrough#implicit-switch-case-fall-through



Signed-off-by: Ruan Jinjie <ruanjinjie@huawei.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: linux-kernel@vger.kernel.org