x86/entry/64: Move ENTER_IRQ_STACK from interrupt macro to interrupt_entry

Moving the switch to IRQ stack from the interrupt macro to the helper
function requires some trickery: All ENTER_IRQ_STACK really cares about
is where the "original" stack -- meaning the GP registers etc. -- is
stored. Therefore, we need to offset the stored RSP value by 8 whenever
ENTER_IRQ_STACK is called from within a function. In such cases, and
after switching to the IRQ stack, we need to push the "original" return
address (i.e. the return address from the call to the interrupt entry
function) to the IRQ stack.

This trickery allows us to carve another .85k from the text size (it
would be more except for the additional unwind hints):

   text	   data	    bss	    dec	    hex	filename
  18006	      0	      0	  18006	   4656	entry_64.o-orig
  17158	      0	      0	  17158	   4306	entry_64.o

Signed-off-by: Dominik Brodowski <linux@dominikbrodowski.net>
Acked-by: Thomas 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: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: dan.j.williams@intel.com
Link: http://lkml.kernel.org/r/20180220210113.6725-3-linux@dominikbrodowski.net
Signed-off-by: Ingo Molnar <mingo@kernel.org>