Fix: perform TLS fixup in all UST entry points from each thread

Each entry point into lttng-ust that end up taking the ust lock need to
perform a TLS fixup for each thread. Add a TLS fixup in both listener
threads, in fork and base address dump helper libs, and in app context
and tracepoint probe registration/unregistration functions, which can be
called from application threads.

Those ensure we don't take the libc dl lock within the ust lock when
performing the TLS lazy fixup.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>