Changes reset trigger for linux.thread-reset and linux.clone-reset tests to -reset_at_nth_thread. To increase coverage of these tests, we need to trigger reset at a time when multiple threads are active. This is difficult to do using the existing -reset_at_fragment_count option in tests, which may need to be tuned regularly to its ideal value, which also differs based on platform.
Implements some missing AArch64 pieces for dynamorio_sigreturn.
Relaxes dispatch assert to allow threads to enter dispatch after reset. The current condition allows it only when sending threads native.
Fixes #4496 (closed) Fixes #4497 (closed)