flambda-backend: Implement 5.x atomic primitives in Flambda 2 and use them in the stdlib (#2060)

* Implement the atomic primitives in Flambda 2

* Use the atomic primitives in stdlib/atomic.ml

* Use the atomic primitives in stdlib/stdlib.ml

Author: mshinwell

stdlib/atomic.ml

@@ -14,19 +14,11 @@
 
 type !'a t
 
-(* BACKPORT BEGIN switch to primtiives
 external make : 'a -> 'a t = "%makemutable"
 external get : 'a t -> 'a = "%atomic_load"
 external exchange : 'a t -> 'a -> 'a = "%atomic_exchange"
 external compare_and_set : 'a t -> 'a -> 'a -> bool = "%atomic_cas"
 external fetch_and_add : int t -> int -> int = "%atomic_fetch_add"
-*)
-external make : 'a -> 'a t = "caml_atomic_make"
-external get : 'a t -> 'a = "caml_atomic_load"
-external exchange : 'a t -> 'a -> 'a = "caml_atomic_exchange"
-external compare_and_set : 'a t -> 'a -> 'a -> bool = "caml_atomic_cas"
-external fetch_and_add : int t -> int -> int = "caml_atomic_fetch_add"
-(* BACKPORT END *)
 
 external ignore : 'a -> unit = "%ignore"
 

stdlib/stdlib.ml

@@ -557,17 +557,10 @@ external sys_exit : int -> 'a = "caml_sys_exit"
 
 (* for at_exit *)
 type 'a atomic_t
-(* BACKPORT BEGIN
 external atomic_make : 'a -> 'a atomic_t = "%makemutable"
 external atomic_get : 'a atomic_t -> 'a = "%atomic_load"
 external atomic_compare_and_set : 'a atomic_t -> 'a -> 'a -> bool
   = "%atomic_cas"
-*)
-external atomic_make : 'a -> 'a atomic_t = "caml_atomic_make"
-external atomic_get : 'a atomic_t -> 'a = "caml_atomic_load"
-external atomic_compare_and_set : 'a atomic_t -> 'a -> 'a -> bool
-  = "caml_atomic_cas"
-(* BACKPORT END *)
 
 let exit_function = atomic_make flush_all