x86/entry: Reduce the code footprint of the 'idtentry' macro

Play a little trick in the generic PUSH_AND_CLEAR_REGS macro
to insert the GP registers "above" the original return address.

This allows us to (re-)insert the macro in error_entry() and
paranoid_entry() and to remove it from the idtentry macro. This
reduces the static footprint significantly:

   text	   data	    bss	    dec	    hex	filename
  24307	      0	      0	  24307	   5ef3	entry_64.o-orig
  20987	      0	      0	  20987	   51fb	entry_64.o

Co-developed-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Dominik Brodowski <linux@dominikbrodowski.net>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Arjan van de Ven <arjan@linux.intel.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: David Woodhouse <dwmw2@infradead.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/20180214175924.23065-2-linux@dominikbrodowski.net


[ Small tweaks to comments. ]
Signed-off-by: Ingo Molnar <mingo@kernel.org>