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: NAndi Kleen <ak@linux.intel.com>
Signed-off-by: NAdrian 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.comSigned-off-by: NArnaldo Carvalho de Melo <acme@redhat.com>