perf symbols: Try the .debug/ DSO cache as a last resort

perf symbols: Try the .debug/ DSO cache as a last resort

Not as the first attempt at finding a vmlinux for the running kernel,
this way we get a more informative filename to present in tools, it will
check that the build-id is the same as the one previously loaded in the
DSO in dso->build_id, reading from /sys/kernel/notes, for instance.

E.g. in the annotation TUI, going from 'perf top', for the scsi_sg_alloc
kernel function, in the first line:

Before:

scsi_sg_alloc  /root/.debug/.build-id/28/2777c262e6b3c0451375163c9a81c893218ab1

After:

scsi_sg_alloc  /lib/modules/4.3.0-rc1+/build/vmlinux

And:

  # ls -la /root/.debug/.build-id/28/2777c262e6b3c0451375163c9a81c893218ab1
lrwxrwxrwx. 1 root root 81 Sep 22 16:11 /root/.debug/.build-id/28/2777c262e6b3c0451375163c9a81c893218ab1 -> ../../home/git/build/v4.3.0-rc1+/vmlinux/282777c262e6b3c0451375163c9a81c893218ab1
  # file ~/.debug/home/git/build/v4.3.0-rc1+/vmlinux/282777c262e6b3c0451375163c9a81c893218ab1
/root/.debug/home/git/build/v4.3.0-rc1+/vmlinux/282777c262e6b3c0451375163c9a81c893218ab1: ELF 64-bit LSB executable, x86-64, version 1 (SYSV), statically linked, BuildID[sha1]=282777c262e6b3c0451375163c9a81c893218ab1, not stripped
  #

The same as:

  # file /lib/modules/4.3.0-rc1+/build/vmlinux
/lib/modules/4.3.0-rc1+/build/vmlinux: ELF 64-bit LSB executable, x86-64, version 1 (SYSV), statically linked, BuildID[sha1]=282777c262e6b3c0451375163c9a81c893218ab1, not stripped

Furthermore:

  # sha256sum /lib/modules/4.3.0-rc1+/build/vmlinux
  e7a789bbdc61029ec09140c228e1dd651271f38ef0b8416c0b7d5ff727b98be2  /lib/modules/4.3.0-rc1+/build/vmlinux
  # sha256sum ~/.debug/home/git/build/v4.3.0-rc1+/vmlinux/282777c262e6b3c0451375163c9a81c893218ab1
  e7a789bbdc61029ec09140c228e1dd651271f38ef0b8416c0b7d5ff727b98be2  /root/.debug/home/git/build/v4.3.0-rc1+/vmlinux/282777c262e6b3c0451375163c9a81c893218ab1
  [root@zoo new]#

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Borislav Petkov <bp@suse.de>
Cc: David Ahern <dsahern@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Stephane Eranian <eranian@google.com>
Cc: Wang Nan <wangnan0@huawei.com>
Link: http://lkml.kernel.org/n/tip-9y42ikzq3jisiddoi6f07n8z@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>