selftests/seccomp: mips: Remove O32-specific macro

Instead of having the mips O32 macro special-cased, pull the logic into
the SYSCALL_NUM() macro. Additionally include the ABI headers, since
these appear to have been missing, leaving __NR_O32_Linux undefined.

Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/lkml/20200912110820.597135-7-keescook@chromium.org


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