ldso: use pthread_t rather than kernel tid to track ctor visitor

ldso: use pthread_t rather than kernel tid to track ctor visitor

commit 188759bbee057aa94db2bbb7cf7f5855f3b9ab53 documented the intent
to allow recursive dlopen based on tracking ctor_visitor, but used a
kernel tid rather than the pthread_t to identify the caller. as a
result, it would not behave as intended under fork by a ctor, where
the child tid would not match.