Stop single stepping in signal handers on arm64

We should clear the single step flag when entering a signal hander and
set it when returning. This fixes the ptrace__PT_STEP_with_signal test.

While here add support for userspace to set the single step bit as on
x86. This can be used by userspace for self tracing.

Reviewed by: kib
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D34170