Fix: liblttng-ust-libc-wrapper recursive use of calloc

Fix: liblttng-ust-libc-wrapper recursive use of calloc

When built with URCU with --disable-compiler-tls, URCU tls-compat.h uses
calloc in its TLS compat layer. This is used by RCU read-side, which is
used by tracepoint. So using a tracepoint in calloc triggers an infinite
recursion, since its first use of the TLS (with TLS compat) calls
calloc.

Fix this issue by keeping a per-thread nesting counter, and as soon as
we are allocating memory from a context nested within the memory
allocator, don't trace.

Don't trace from the static allocator: keep it as simple as possible,
since this is our fallback.

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