flambda-backend: Add a returns field to Cmm.Cextcall (#74)

Author: xclerc

testsuite/tools/parsecmm.mly

@@ -223,6 +223,7 @@ expr:
   | LPAREN EXTCALL STRING exprlist machtype RPAREN
                {Cop(Cextcall {name=$3; ret=$5; alloc=false;
                               builtin=false;
+                              returns=true;
                               effects=Arbitrary_effects;
                               coeffects=Has_coeffects;
                               ty_args=[];},