Fix: baddr deadlocks and RCU races

Fix deadlocks wrt internal libc mutexes.

Fix RCU races by ensuring every UST source file using RCU read-side lock
is marked as _LGPL_SOURCE. The dynamic binding to urcu-bp relies on the
dynamic loader, and it seems not to behave well when used from
constructors.

Use Userspace RCU with this commit to completely fix baddr races:

"Fix: urcu-bp interaction with threads vs constructors/destructors"

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