fix flush err_formatter issue

Note this can not be done  in the patch of Format.ml,
since Format module is not snapshot, so the benefit will not trickle down to non-patched compilers (including windows)

Author: bobzhang

jscomp/core/bs_conditional_initial.ml

@@ -70,3 +70,5 @@ let setup_env () =
   ; Switch.cut := 100 (* tweakable but not very useful *)
 #end  
 
+let () = 
+    at_exit (fun _ -> Format.pp_print_flush Format.err_formatter ())

lib/4.06.1/unstable/js_compiler.ml

@@ -81628,7 +81628,8 @@ let setup_env () =
   Lexer.replace_directive_string "BS_VERSION"  Bs_version.version
 
 
-
+let () = 
+    at_exit (fun _ -> Format.pp_print_flush Format.err_formatter ())
 end
 module Ext_pervasives : sig 
 #1 "ext_pervasives.mli"

lib/4.06.1/unstable/js_refmt_compiler.ml

@@ -81628,7 +81628,8 @@ let setup_env () =
   Lexer.replace_directive_string "BS_VERSION"  Bs_version.version
 
 
-
+let () = 
+    at_exit (fun _ -> Format.pp_print_flush Format.err_formatter ())
 end
 module Ext_pervasives : sig 
 #1 "ext_pervasives.mli"

lib/4.06.1/whole_compiler.ml

@@ -365478,7 +365478,8 @@ let setup_env () =
   Lexer.replace_directive_string "BS_VERSION"  Bs_version.version
 
 
-
+let () = 
+    at_exit (fun _ -> Format.pp_print_flush Format.err_formatter ())
 end
 module Ext_spec : sig 
 #1 "ext_spec.mli"