Tracepoint: make tracepoint ptrs section rw

The pointer section needs to be rw so that the linker can update the
pointers at link-time, thus allowing the code to be compiled with -fPIC.

The previous behavior would cause section flag mismatch, because
TRACEPOINT_LIB declared the ptr with "= NULL", which is not a void *
const, but rather a void *, which forces the element it is assigned to
to become non-const.

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