Add a data pointer to all tracepoint probe callbacks

RCU now dereferences an array of probe structs containing data pointers and
function pointers, and passes the data pointer as the first argument to the
probe.

Due to this change void prototypes are no longer allowed, which has incurred
changes to libust-initializer.c .

This patch also fixes a problem where non-existent tracepoints are present
in the __tracepoints section and with a name pointer pointing at NULL.