fix asm_expr_unary()

keep unary unsigned.
    problem is exposed on i386 with unary like 0xC0000000.
    In this case a sign is extended in
        pe->v = n;
    if n declared as long.

tccasm.c

@@ -42,7 +42,7 @@ static void asm_expr_unary(TCCState *s1, ExprValue *pe)
 {
     Sym *sym;
     int op, label;
-    long n;
+    unsigned long n;
     const char *p;
 
     switch(tok) {