perf/urgent fixes:

- Fix probing of precise_ip level for default cycles event, that
  got broken recently on x86_64 when its arch code started
  considering invalid requesting precise samples when not sampling
  (i.e. when attr.sample_period == 0).

  This also fixes another problem in s/390 where the precision
  probing with sample_period == 0 returned precise_ip > 0, that
  then, when setting up the real cycles event (not probing) would
  return EOPNOTSUPP for precise_ip > 0 (as determined previously
  by probing) and sample_period > 0.

  These problems resulted in attr_precise not being set to the
  highest precision available on x86.64 when no event was specified,
  i.e. the canonical:

	perf record ./workload

  would end up using attr.precise_ip = 0. As a workaround this would
  need to be done:

	perf record -e cycles:P ./workload

  And on s/390 it would plain not work, requiring using:

        perf record -e cycles ./workload

  as a workaround.  (Arnaldo Carvalho de Melo)

- Fix perf build with ARCH=x86_64, when ARCH should be transformed
  into ARCH=x86, just like with the main kernel Makefile and
  tools/objtool's, i.e. use SRCARCH. (Jiada Wang)

- Avoid accessing uninitialized data structures when unwinding with
  elfutils's libdw, making it more closely mimic libunwind's unwinder.
  (Milian Wolff)

Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
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