selftests/seccomp: powerpc: Set syscall return during ptrace syscall exit

Some archs (like powerpc) only support changing the return code during
syscall exit when ptrace is used. Test entry vs exit phases for which
portions of the syscall number and return values need to be set at which
different phases. For non-powerpc, all changes are made during ptrace
syscall entry, as before. For powerpc, the syscall number is changed at
ptrace syscall entry and the syscall return value is changed on ptrace
syscall exit.

Reported-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>
Suggested-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>
Link: https://lore.kernel.org/linux-kselftest/20200911181012.171027-1-cascardo@canonical.com/


Fixes: 58d0a862 ("seccomp: add tests for ptrace hole")
Acked-by: Christian Brauner <christian.brauner@ubuntu.com>
Link: https://lore.kernel.org/lkml/20200921075300.7iylzof2w5vrutah@wittgenstein/


Signed-off-by: Kees Cook <keescook@chromium.org>