Add __builtin_frame_address(0)

Adding the GCC extension __builtin_frame_address(). We support only zero as the argument for now.
With this functionality, we can implement GCC compatible stdarg by macros in x86-64.

tcc.c

@@ -7599,6 +7599,24 @@ static void unary(void)
             vpushi(res);
         }
         break;
+    case TOK_builtin_frame_address:
+        {
+            CType type;
+            next();
+            skip('(');
+            if (tok != TOK_CINT) {
+                error("__builtin_frame_address only takes integers");
+            }
+            if (tokc.i != 0) {
+                error("TCC only supports __builtin_frame_address(0)");
+            }
+            next();
+            skip(')');
+            type.t = VT_VOID;
+            mk_pointer(&type);
+            vset(&type, VT_LOCAL, 0);
+        }
+        break;
     case TOK_INC:
     case TOK_DEC:
         t = tok;

tcctok.h

@@ -109,6 +109,7 @@
      DEF(TOK_NORETURN2, "__noreturn__")
      DEF(TOK_builtin_types_compatible_p, "__builtin_types_compatible_p")
      DEF(TOK_builtin_constant_p, "__builtin_constant_p")
+     DEF(TOK_builtin_frame_address, "__builtin_frame_address")
      DEF(TOK_REGPARM1, "regparm")
      DEF(TOK_REGPARM2, "__regparm__")