flambda-backend: Add flambda_oclassic attribute (ocaml#348)

Author: mshinwell

parsing/builtin_attributes.ml

@@ -342,6 +342,12 @@ let nolabels_attribute attr =
   clflags_attribute_without_payload attr
     ~name:"nolabels" Clflags.classic
 
+let flambda_oclassic_attribute attr =
+  clflags_attribute_without_payload' attr
+    ~name:"flambda_oclassic"
+    ~f:(fun () ->
+      if Config.flambda || Config.flambda2 then Clflags.set_oclassic ())
+
 let flambda_o3_attribute attr =
   clflags_attribute_without_payload' attr
     ~name:"flambda_o3"
@@ -378,4 +384,5 @@ let parse_standard_implementation_attributes attr =
   nolabels_attribute attr;
   inline_attribute attr;
   afl_inst_ratio_attribute attr;
-  flambda_o3_attribute attr
+  flambda_o3_attribute attr;
+  flambda_oclassic_attribute attr

parsing/builtin_attributes.mli

@@ -31,6 +31,7 @@
     - ocaml.inline
     - ocaml.afl_inst_ratio
     - ocaml.flambda_o3
+    - ocaml.flambda_oclassic
 
     {b Warning:} this module is unstable and part of
   {{!Compiler_libs}compiler-libs}.
@@ -88,4 +89,4 @@ val has_unboxed: Parsetree.attributes -> bool
 val has_boxed: Parsetree.attributes -> bool
 
 val parse_standard_interface_attributes : Parsetree.attribute -> unit
-val parse_standard_implementation_attributes : Parsetree.attribute -> unit
+val parse_standard_implementation_attributes : Parsetree.attribute -> unit