modula2: Ensure that module registration constructors are 'extern' [PR108183].

The symbols for module registration constructors need to be external
or we get wrong code generated for targets that allow direct access to
local symbol definitions.

Signed-off-by: Iain Sandoe <iain@sandoe.co.uk>

	PR modula2/108183

gcc/m2/ChangeLog:

	* gm2-compiler/M2GCCDeclare.mod: Module registration constructors are
	externs to the builder of m2_link.

Co-Authored-By: Gaius Mulley <gaiusmod2@gmail.com>

gcc/m2/gm2-compiler/M2GCCDeclare.mod

@@ -2294,6 +2294,11 @@ PROCEDURE IsExternal (sym: CARDINAL) : BOOLEAN ;
 VAR
    mod: CARDINAL ;
 BEGIN
+   Assert (NOT IsDefImp (sym)) ;
+   IF IsProcedure (sym) AND IsExtern (sym)
+   THEN
+     RETURN TRUE
+   END ;
    mod := GetScope(sym) ;
    REPEAT
       IF mod=NulSym