Change Intel PT's branch stack support to use thread stacks. The
advantages of using branch stack support from the thread-stack are:

1. the branches are accumulated separately for each thread
2. the branch stack is cleared only in between continuous traces

This helps pave the way for adding branch stacks to regular events, not
just synthesized events as at present.

While the 2 approaches are not identical, in simple cases the results
can be identical e.g.

  Before:

    # perf record --kcore -e intel_pt// uname
    # perf script --itrace=i10usl -F+brstacksym,+addr,+flags > cmp1.txt

  After:

    # perf script --itrace=i10usl -F+brstacksym,+addr,+flags > cmp2.txt
    # diff -s cmp1.txt cmp2.txt
    Files cmp1.txt and cmp2.txt are identical
Signed-off-by: NAdrian Hunter <adrian.hunter@intel.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Link: http://lore.kernel.org/lkml/20200429150751.12570-4-adrian.hunter@intel.comSigned-off-by: NArnaldo Carvalho de Melo <acme@redhat.com>