selftests/seccomp: add MIPS self-test support

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 <[email protected]>
Acked-by: Kees Cook <[email protected]>
Cc: Andy Lutomirski <[email protected]>
Cc: Shuah Khan <[email protected]>
Cc: Will Drewry <[email protected]>
Cc: [email protected]
Cc: [email protected]
Cc: [email protected]
Patchwork: https://patchwork.linux-mips.org/patch/12977/
Signed-off-by: Ralf Baechle <[email protected]>