Fix: trace events in C++ constructors/destructors

Fix: trace events in C++ constructors/destructors

Wrap constructor and destructor functions to invoke them as functions with
the constructor/destructor GNU C attributes, which ensures that those
constructors/destructors are ordered before/after C++
constructors/destructors.

Wrap constructor and destructor functions as the constructor/destructor of a
variable defined within an anonymous namespace when building as C++ with
LTTNG_UST_ALLOCATE_COMPOUND_LITERAL_ON_HEAP defined. With this option,
there are no guarantees that the events in C++ constructors/destructors will
be traced.

Fixes: 05bfa3dc3a6e ("Fix: generate probe registration constructor as a C++ constuctor")
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Change-Id: If058b15af6b4d8852fa29d0a21b8233bcb4b43a2