x86/entry/64: Fix unwind hints in __switch_to_asm()

UNWIND_HINT_FUNC has some limitations: specifically, it doesn't reset
all the registers to undefined.  This causes objtool to get confused
about the RBP push in __switch_to_asm(), resulting in bad ORC data.

While __switch_to_asm() does do some stack magic, it's otherwise a
normal callable-from-C function, so just annotate it as a function,
which makes objtool happy and allows it to produces the correct hints
automatically.

Fixes: 8c1f7558 ("x86/entry/64: Add unwind hint annotations")
Reviewed-by: NMiroslav Benes <mbenes@suse.cz>
Signed-off-by: NJosh Poimboeuf <jpoimboe@redhat.com>
Signed-off-by: NIngo Molnar <mingo@kernel.org>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Dave Jones <dsj@fb.com>
Cc: Jann Horn <jannh@google.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Vince Weaver <vincent.weaver@maine.edu>
Link: https://lore.kernel.org/r/03d0411920d10f7418f2e909210d8e9a3b2ab081.1587808742.git.jpoimboe@redhat.com

arch/x86/entry/entry_64.S

@@ -279,8 +279,7 @@ SYM_CODE_END(entry_SYSCALL_64)
  * %rdi: prev task
  * %rsi: next task
  */
-SYM_CODE_START(__switch_to_asm)
-	UNWIND_HINT_FUNC
+SYM_FUNC_START(__switch_to_asm)
 	/*
 	 * Save callee-saved registers
 	 * This must match the order in inactive_task_frame
@@ -321,7 +320,7 @@ SYM_CODE_START(__switch_to_asm)
 	popq	%rbp
 
 	jmp	__switch_to
-SYM_CODE_END(__switch_to_asm)
+SYM_FUNC_END(__switch_to_asm)
 
 /*
  * A newly forked process directly context switches into this address.