linux-user: Create special-errno.h

Pull the two internal errno used by qemu internally into their own
header file.  This includes the one define required by safe-syscall.S.

Reviewed-by: Warner Losh <imp@bsdimp.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>

linux-user/syscall.c

@@ -136,6 +136,7 @@
 #include "qemu/guest-random.h"
 #include "qemu/selfmap.h"
 #include "user/syscall-trace.h"
+#include "special-errno.h"
 #include "qapi/error.h"
 #include "fd-trans.h"
 #include "tcg/tcg.h"