selftests/seccomp: Refactor arch register macros to avoid xtensa special case

To avoid an xtensa special-case, refactor all arch register macros to
take the register variable instead of depending on the macro expanding
as a struct member name.

Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/lkml/20200912110820.597135-2-keescook@chromium.org


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