[APX] Support Intel APX PUSH2POP2

This feature requires stack to be aligned at 16byte, therefore in
prologue/epilogue, a standalone push/pop will be emitted before any
push2/pop2 if the stack was not aligned to 16byte.
Also for current implementation we only support push2/pop2 usage in
function prologue/epilogue for those callee-saved registers.

gcc/ChangeLog:

	* config/i386/i386.cc (gen_push2): New function to emit push2
	and adjust cfa offset.
	(ix86_pro_and_epilogue_can_use_push2_pop2): New function to
	determine whether push2/pop2 can be used.
	(ix86_compute_frame_layout): Adjust preferred stack boundary
	and stack alignment needed for push2/pop2.
	(ix86_emit_save_regs): Emit push2 when available.
	(ix86_emit_restore_reg_using_pop2): New function to emit pop2
	and adjust cfa info.
	(ix86_emit_restore_regs_using_pop2): New function to loop
	through the saved regs and call above.
	(ix86_expand_epilogue): Call ix86_emit_restore_regs_using_pop2
	when push2pop2 available.
	* config/i386/i386.md (push2_di): New pattern for push2.
	(pop2_di): Likewise for pop2.

gcc/testsuite/ChangeLog:

	* gcc.target/i386/apx-push2pop2-1.c: New test.
	* gcc.target/i386/apx-push2pop2_force_drap-1.c: Likewise.
	* gcc.target/i386/apx-push2pop2_interrupt-1.c: Likewise.

Co-authored-by: Hu Lin1 <lin1.hu@intel.com>
Co-authored-by: Hongyu Wang <hongyu.wang@intel.com>