[ARC] Propagate uncached type attribute to each member of a struct.

Like `packed` type attribute, the ARC's `uncached` type attribute
needs to be propagated to each member of the struct where it is used,
triggering the .di flag for any access of the struct members. However,
any complex CFG manipulation may drop memory pointer type attributes,
leading to the impossibility to discriminate the direct accesses from
normal ones. To solve this issue, we will treat the direct memory
accessed specially via unspecs.

gcc/
xxxx-xx-xx  Claudiu Zissulescu  <claziss@synopsys.com>
	Petro Karashchenko  <petro.karashchenko@ring.com>

	* config/arc/arc.c (arc_is_uncached_mem_p): Check struct
	attributes if needed.
	(prepare_move_operands): Generate special
	unspec instruction for direct access.
	(arc_isuncached_mem_p): Propagate uncached attribute to each
	structure member.
	* config/arc/arc.md (VUNSPEC_ARC_LDDI): Define.
	(VUNSPEC_ARC_STDI): Likewise.
	(ALLI): New mode iterator.
	(mALLI): New mode attribute.
	(lddi): New instruction pattern.
	(stdi): Likewise.
	(stdidi_split): Split instruction for architectures which are not
	supporting ll64 option.
	(lddidi_split): Likewise.

testsuite/
xxxx-xx-xx  Claudiu Zissulescu  <claziss@synopsys.com>
	Petro Karashchenko  <petro.karashchenko@ring.com>

	* gcc.target/arc/uncached-1.c: Update test.
	* gcc.target/arc/uncached-2.c: Likewise.
	* gcc.target/arc/uncached-3.c: New test.
	* gcc.target/arc/uncached-4.c: Likewise.
	* gcc.target/arc/uncached-5.c: Likewise.
	* gcc.target/arc/uncached-6.c: Likewise.
	* gcc.target/arc/uncached-7.c: Likewise.
	* gcc.target/arc/uncached-8.c: Likewise.
	* gcc.target/arc/arc.exp (ll64): New predicate.