Fix: fix deadlock with dlopen() lttng-ust

Pre-"fault" the libuuid TLS outside of the ust lock to ensure that we
never deadlock with the linker mutex in libc. This mutex is held both
for TLS fixup of dlopen'd libs, and across execution of all
constructors. We therefore have a reverse use of this mutex wrt ust
lock, that we fix by forcing the TLS fixup to happen very early in the
lttng-ust constructor.

[fixed merge from dev branch]

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