[ARC] Prevent moving stores to the frame before the stack adjustment.

[ARC] Prevent moving stores to the frame before the stack adjustment.

If the stack pointer is needed, emit a special barrier that will prevent
the scheduler from moving stores to the frame before the stack adjustment.

For example:

[snip]
	mov_s	fp,sp 	; frame pointer is set here
[snip]
	st	r1,[fp,-24]	; frame pointer is used here
[snip]
	sub_s	sp,sp,0x20	; stack pointer adjusted

So we can easily see that any interrupt between the `st` and `sub`
instruction will lead to faulty code as the interrupt routine will use
a faulty sp register, and, potentially, overwriting the value stored
by 'st' instruction. Thus, adding a scheduler barrier will force the
compiler to emit the `sub` instruction before the store one.

2017-06-01  Claudiu Zissulescu  <claziss@synopsys.com>

	* config/arc/arc.c (arc_expand_prologue): Emit a special barrier
	to prevent store reordering.
	* config/arc/arc.md (UNSPEC_ARC_STKTIE): Define.
	(type): Add block type.
	(stack_tie): Define special instruction to be used in
	expand_prologue.

From-SVN: r248781