selftests/seccomp: add MIPS self-test support

This adds self-test support on MIPS, based on RFC patch from Kees Cook.
Modifications from the RFC:
- support the O32 syscall which passes the real syscall number in a0.
- Use PTRACE_{GET,SET}REGS
- Because SYSCALL_NUM and SYSCALL_RET are the same register, it is not
  possible to test modifying the syscall return value when skipping,
  since both would need to set the same register. Therefore modify that
  test case to just detect the skipped test.
Tested on MIPS32r2 / MIPS64r2 with O32, N32 and N64 userlands.

Signed-off-by: Matt Redfearn <matt.redfearn@imgtec.com>
Acked-by: Kees Cook <keescook@chromium.org>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Shuah Khan <shuahkh@osg.samsung.com>
Cc: Will Drewry <wad@chromium.org>
Cc: IMG-MIPSLinuxKerneldevelopers@imgtec.com
Cc: linux-kernel@vger.kernel.org
Cc: linux-kselftest@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/12977/


Signed-off-by: Ralf Baechle <ralf@linux-mips.org>