selftests/seccomp: build and pass on arm64

Changing arm64 syscalls is done via a specific register set, more like s390
than like arm (specific ptrace call) and x86 (part of general registers).
Since (restarting) poll doesn't exist on arm64, switch to using nanosleep
for testing restart_syscall. And since it looks like the syscall ABI is
inconsistent on arm-compat, so we must work around it (and document it) in
the test.

Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>