selftests/seccomp: Handle arm32 corner cases better

It turns out arm32 doesn't handle syscall -1 gracefully, so skip testing
for that. Additionally skip tests that depend on clone3 when it is not
available (for example when building the seccomp selftests on an old arm
image without clone3 headers). And improve error reporting for when
nanosleep fails, as seen on arm32 since v5.15.

Cc: Lecopzer Chen <lecopzer.chen@mediatek.com>
Signed-off-by: Kees Cook <keescook@chromium.org>