objtool: Allow UNWIND_HINT to suppress dodgy stack modifications

rewind_stack_do_exit()
	UNWIND_HINT_FUNC
	/* Prevent any naive code from trying to unwind to our caller. */

	xorl	%ebp, %ebp
	movq	PER_CPU_VAR(cpu_current_top_of_stack), %rax
	leaq	-PTREGS_SIZE(%rax), %rsp
	UNWIND_HINT_REGS

	call	do_exit

Does unspeakable things to the stack, which objtool currently fails to
detect due to a limitation in instruction decoding. This will be
rectified after which the above will result in:

arch/x86/entry/entry_64.o: warning: objtool: .text+0xab: unsupported stack register modification

Allow the UNWIND_HINT on the next instruction to suppress this, it
will overwrite the state anyway.

Suggested-by: Josh Poimboeuf <jpoimboe@redhat.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Acked-by: Josh Poimboeuf <jpoimboe@redhat.com>
Tested-by: Nick Desaulniers <ndesaulniers@google.com>
Link: https://lkml.kernel.org/r/20210211173626.918498579@infradead.org