selftests/seccomp: Convert REGSET calls into ARCH_GETREG/ARCH_SETREG

Consolidate the REGSET logic into the new ARCH_GETREG() and
ARCH_SETREG() macros, avoiding more #ifdef code in function bodies.

Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/lkml/20200912110820.597135-10-keescook@chromium.org


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