Fix dynamic linking with GNU gold linker

Fix dynamic linking with GNU gold linker

Programs that were not linked explicitely with -lurcu-bp generated no
trace output when linked with GNU gold, because the urcu-bp library
symbols were not used to populate the weak urcu-bp symbols used by the
program. (even though they worked fine with the standard GNU ld).

Solve this by creating those wrapper symbols into tracepoint.c, and
explicitely get them with dlsym() in the constructor.

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