x86: Remove useless unwinder backlink from irq regs saving

The unwinder backlink in interrupt entry is very useless.
It's actually not part of the stack frame chain and thus is
never used.

Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Jan Beulich <JBeulich@novell.com>

arch/x86/kernel/entry_64.S

@@ -327,7 +327,6 @@ ENDPROC(native_usergs_sysret64)
 	jne 2f
 	mov PER_CPU_VAR(irq_stack_ptr),%rsp
 	EMPTY_FRAME 0
-	pushq_cfi %rbp			/* backlink for unwinder */
 	/*
 	 * We entered an interrupt context - irqs are off:
 	 */