drop alloca #define

(Because GNU's alloca.h unconditionally #undef's alloca)

Also, remove gcc specific sections in headers. and
instead change tests such that gcc does not use them.

libtcc.c

@@ -694,7 +694,7 @@ static void put_extern_sym2(Sym *sym, Section *section,
             case TOK_memset:
             case TOK_strlen:
             case TOK_strcpy:
-            case TOK__alloca:
+            case TOK_alloca:
                 strcpy(buf, "__bound_");
                 strcat(buf, name);
                 name = buf;