[utils/threads] Export small ID functions as part of the API.

[utils/threads] Export small ID functions as part of the API.

These will be used in the log profiler.

Also, make mono_thread_info_register_small_id () only allocate a new small ID
if one isn't already allocated for the current thread. This is necessary so
that we can allocate a small ID for a thread and not have it be overwritten
if/when that thread is later attached probably (which also results in this
function being called).