pipe argument should not be signed

pipedes is an address, it should not be signed (breaks for addresses
> 0x80000000)
Signed-off-by: NUlrich Hecht <uli@suse.de>
Signed-off-by: NRiku Voipio <riku.voipio@iki.fi>

linux-user/syscall.c

@@ -953,7 +953,7 @@ static abi_long do_pipe2(int host_pipe[], int flags)
 #endif
 }
 
-static abi_long do_pipe(void *cpu_env, int pipedes, int flags)
+static abi_long do_pipe(void *cpu_env, abi_ulong pipedes, int flags)
 {
     int host_pipe[2];
     abi_long ret;