os::current_thread_id() is not signal safe on macOS

On macOS, os::current_thread_id() and os::Bsd::gettid() are implement as
pthread_mach_thread_np(pthread_self())

However, pthread_mach_thread_np() is not async signal safe. Despite this fact, pthread_mach_thread_np() is still used inside a signal handler for error reporting. Because of that, the process may hang during error reporting or crash without producing hs_err_pid.log.

The problem was noticed when investigating a crash in async-profiler, see https://github.com/jvm-profiling-tools/async-profiler/issues/242

The following code is an alternative for os::current_thread_id():

    mach_port_t port = mach_thread_self();
    mach_port_deallocate(mach_task_self(), port);
    return (int)port;

It produces the same result as pthread_mach_thread_np(pthread_self()), but is safe to use inside a signal handler.