perf intel-pt: Support generating branch stack

Add support for generating branch stack context for PT samples.  The
decoder reports a configurable number of branches as branch context for
each sample. Internally it keeps track of them by using a simple sliding
window.  We also flush the last branch buffer on each sample to avoid
overlapping intervals.

This is useful for:

- Reporting accurate basic block edge frequencies through the perf
  report branch view
- Using with --branch-history to get the wider context of samples
- Other users of LBRs

Also the Documentation is updated.

Examples:

	Record with Intel PT:

		perf record -e intel_pt//u ls

	Branch stacks are used by default if synthesized so:

		perf report --itrace=ile

	is the same as:

		perf report --itrace=ile -b

	Branch history can be requested also:

		perf report --itrace=igle --branch-history

Based-on-patch-by: Andi Kleen <ak@linux.intel.com>
Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Link: http://lkml.kernel.org/r/1443186956-18718-15-git-send-email-adrian.hunter@intel.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/Documentation/intel-pt.txt

@@ -671,6 +671,7 @@ The letters are:
 	e	synthesize tracing error events
 	d	create a debug log
 	g	synthesize a call chain (use with i or x)
+	l	synthesize last branch entries (use with i or x)
 
 "Instructions" events look like they were recorded by "perf record -e
 instructions".
@@ -718,6 +719,15 @@ transactions events can be specified. e.g.
 	--itrace=ig32
 	--itrace=xg32
 
+Also the number of last branch entries (default 64, max. 1024) for instructions or
+transactions events can be specified. e.g.
+
+       --itrace=il10
+       --itrace=xl10
+
+Note that last branch entries are cleared for each sample, so there is no overlap
+from one sample to the next.
+
 To disable trace decoding entirely, use the option --no-itrace.