selftests/seccomp: mips: Define SYSCALL_NUM_SET macro

Remove the mips special-case in change_syscall().

Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/lkml/20200912110820.597135-4-keescook@chromium.org


Acked-by: Christian Brauner <christian.brauner@ubuntu.com>