selftests/seccomp: sh: Fix register names

It looks like the seccomp selftests was never actually built for sh.
This fixes it, though I don't have an environment to do a runtime test
of it yet.

Fixes: 0bb605c2

 ("sh: Add SECCOMP_FILTER")
Tested-by: John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>
Link: https://lore.kernel.org/lkml/a36d7b48-6598-1642-e403-0c77a86f416d@physik.fu-berlin.de
Signed-off-by: Kees Cook <keescook@chromium.org>