perf unwind: Fix looking up dwarf unwind stack info

perf unwind: Fix looking up dwarf unwind stack info

Using perf with call graph method dwarf fails to provide backtrace
support for stripped binary even though .gnu_debuglink points to *.dbg
flavor with properly populated debug symbols.

Problem is reproduced on ARM (v7, v8), kernels 3.14.y, 4.4.y and
4.10.rc3.  Perf is configured with libunwind, and unwind dwarf support
[1]. Test code (stress_bt.c) can be found on [2].

Running (explicitly disable other unwinding methods):

  $ gcc -g -o stress_bt -fomit-frame-pointer -fno-unwind-tables \
	-fno-asynchronous-unwind-tables stress_bt.c
  $ perf record -N --call-graph dwarf ./stress_bt
  $ perf report

results in properly generated call graph. Stripping the binary and running
it results with missing call graph. Expected result is to have call graph:

  $ gcc -g -o stress_bt -fomit-frame-pointer -fno-unwind-tables \
	  -fno-asynchronous-unwind-tables stress_bt.c
  $ objcopy --only-keep-debug stress_bt stress_bt.dbg
  $ objcopy --strip-debug stress_bt
  $ objcopy --add-gnu-debuglink=stress_bt.dbg stress_bt
  $ perf record -N --call-graph dwarf ./stress_bt
  $ perf report

Problem is that perf doesn't try to read symbols pointed by gnu
debuglink.  Patch adds checking, and reading of the symbols from
debuglink and symsrc.  Order of the check is to first check within dso,
then check whether symsrc is defined and try to read from it. Finally,
debuglink is checked. Default locations of debug files are discussed in
[3] and [4]. Comments on RFC are on [5].

[1] https://wiki.linaro.org/LEG/Engineering/TOOLS/perf-callstack-unwinding
[2] [1]#Backtrace_stress_application
[3] https://sourceware.org/gdb/onlinedocs/gdb/Separate-Debug-Files.html
[4] https://sourceware.org/binutils/docs/binutils/objcopy.html
[5] https://lkml.org/lkml/2016/8/22/473

Signed-off-by: Matija Glavinic Pecotic <matija.glavinic-pecotic.ext@nokia.com>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Cc: Alexander Sverdlin <alexander.sverdlin@nokia.com>
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: http://lkml.kernel.org/r/d309d40a-463f-482b-68e1-1465326efdc1@nokia.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>