tracing/stat handling updates

After a quick look on the need of stats tracers, it looks like it is often needed to have several stat modes/files for a single tracer.

The current tracing stat API doesn't support more than one stat file per tracer.

The first patch makes the tracing/stat Api reentrant and let a tracer able implement more than one stat file.

The second one adapts this Api to the branch tracer.

 kernel/trace/trace.h        |   35 ++++---
 kernel/trace/trace_branch.c |   69 +++++--------
 kernel/trace/trace_stat.c   |  229 +++++++++++++++++++++++++++++++------------
 3 files changed, 216 insertions(+), 117 deletions(-)