tls: Add debug logging for TLS and TCB management

tls: Add debug logging for TLS and TCB management

Introduce the `DL_DEBUG_TLS` debug mask to enable detailed logging for
Thread-Local Storage (TLS) and Thread Control Block (TCB) management.

This change integrates a new `tls` option into the `LD_DEBUG`
environment variable, allowing developers to trace:
- TCB allocation, deallocation, and reuse events in `dl-tls.c`,
  `nptl/allocatestack.c`, and `nptl/nptl-stack.c`.
- Thread startup events, including the TID and TCB address, in
  `nptl/pthread_create.c`.

A new test, `tst-dl-debug-tid`, has been added to validate the
functionality of this new debug logging, ensuring that relevant messages
are correctly generated for both main and worker threads.

This enhances the debugging capabilities for diagnosing issues related
to TLS allocation and thread lifecycle within the dynamic linker.

Reviewed-by: DJ Delorie <dj@redhat.com>