objtool: Add rewind_stack_do_exit() to the noreturn list

This fixes the following warning seen on GCC 7.3:

  arch/x86/kernel/dumpstack.o: warning: objtool: oops_end() falls through to next function show_regs()

Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: https://lkml.kernel.org/r/3418ebf5a5a9f6ed7e80954c741c0b904b67b5dc.1554398240.git.jpoimboe@redhat.com