tccgen.c: fix two scope problems

... see tests

Also:
- tccgen.c: cleanup _Generic a little
- libtcc.c: cleanup mem-debug a little
- arm-link.c: read/write only once
- tcc.h: another fix for clang offsetof
- tccdbg.c:
  * stabs: handle forward struct/enum decls
  * stabs: fix anonymous struct members (must not have a .L123 name)
  * avoid strncpy()

tests/tests2/129_scopes.c

@@ -1,7 +1,11 @@
-#include <stdio.h>
+int printf(const char*, ...);
+
+#define myassert(x) \
+    printf("%s:%d: %s : \"%s\"\n", __FILE__,__LINE__,(x)?"ok":"error",#x)
+
 enum{ in = 0};
-#define myassert(X) do{ if(!X) printf("%d: assertion failed\n", __LINE__); }while(0)
-int main(){
+
+int main_1(){
     {
         myassert(!in);
         if(sizeof(enum{in=1})) myassert(in);
@@ -38,6 +42,45 @@ int main(){
         for(;sizeof(enum{in=1});){ myassert(in); break; }
         myassert(!in); //OK
     }
-
+    return 0;
 }
 
+/* --------------------------------------------- */
+int main_2()
+{
+  char c = 'a';
+  void func1(char c); /* param 'c' must not shadow local 'c' */
+  func1(c);
+  return 0;
+}
+
+void func1(char c)
+{
+    myassert(c == 'a');
+}
+
+struct st { int a; };
+
+/* --------------------------------------------- */
+int main_3()
+{
+  struct st func(void);
+  struct st st = func(); /* not an 'incompatible redefinition' */
+  myassert(st.a == 10);
+  return 0;
+}
+
+struct st func(void)
+{
+  struct st st = { 10 };
+  return st;
+}
+
+/* --------------------------------------------- */
+int main()
+{
+    main_1();
+    main_2();
+    main_3();
+    return 0;
+}

tests/tests2/129_scopes.expect

@@ -0,0 +1,23 @@
+129_scopes.c:10: ok : "!in"
+129_scopes.c:11: ok : "in"
+129_scopes.c:12: ok : "!in"
+129_scopes.c:15: ok : "!in"
+129_scopes.c:16: ok : "in"
+129_scopes.c:17: ok : "!in"
+129_scopes.c:20: ok : "!in"
+129_scopes.c:21: ok : "in"
+129_scopes.c:22: ok : "!in"
+129_scopes.c:25: ok : "!in"
+129_scopes.c:26: ok : "!in"
+129_scopes.c:27: ok : "!in"
+129_scopes.c:31: ok : "!in"
+129_scopes.c:32: ok : "in"
+129_scopes.c:33: ok : "!in"
+129_scopes.c:36: ok : "!in"
+129_scopes.c:37: ok : "in"
+129_scopes.c:38: ok : "!in"
+129_scopes.c:41: ok : "!in"
+129_scopes.c:42: ok : "in"
+129_scopes.c:43: ok : "!in"
+129_scopes.c:59: ok : "c == 'a'"
+129_scopes.c:69: ok : "st.a == 10"