[PATCH] [ARC] Add support for atomic memory built-in.

[Claudiu Zissulescu]

I will add this text before  "*memory_barrier" pattern:

;; For ARCHS, we use a hardware data memory barrier that waits for
;; completion of current data memory operations before initiating
;; similar data memory operations.

Once done, I will commit it.

Thanks,
Claudiu

>
> Tested with dg.exp (when passing -matomic to gcc compiler line, the atomic tests are also successfully executed).
The comment before "*memory_barrier" could use some elaboration on what it does for TARGET_HS.
Otherwise, this is OK.