x86/entry/64: Move PUSH_AND_CLEAR_REGS from interrupt macro to helper function

The PUSH_AND_CLEAR_REGS macro is able to insert the GP registers
"above" the original return address. This allows us to move a sizeable
part of the interrupt entry macro to an interrupt entry helper function:

   text	   data	    bss	    dec	    hex	filename
  21088	      0	      0	  21088	   5260	entry_64.o-orig
  18006	      0	      0	  18006	   4656	entry_64.o
Suggested-by: NLinus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: NDominik Brodowski <linux@dominikbrodowski.net>
Acked-by: NThomas Gleixner <tglx@linutronix.de>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: David Woodhouse <dwmw2@infradead.org>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: dan.j.williams@intel.com
Link: http://lkml.kernel.org/r/20180220210113.6725-2-linux@dominikbrodowski.netSigned-off-by: NIngo Molnar <mingo@kernel.org>

arch/x86/entry/entry_64.S

@@ -525,6 +525,14 @@ END(irq_entries_start)
  *
  * Entry runs with interrupts off.
  */
+ENTRY(interrupt_entry)
+	UNWIND_HINT_FUNC
+
+	PUSH_AND_CLEAR_REGS save_ret=1
+	ENCODE_FRAME_POINTER 8
+
+	ret
+END(interrupt_entry)
 
 /* 0(%rsp): ~(interrupt number) */
 	.macro interrupt func
@@ -536,8 +544,7 @@ END(irq_entries_start)
 	call	switch_to_thread_stack
 1:
 
-	PUSH_AND_CLEAR_REGS
-	ENCODE_FRAME_POINTER
+	call	interrupt_entry
 
 	testb	$3, CS(%rsp)
 	jz	1f