perf symbols: we can now read separate debug-info files based on a build ID

perf symbols: we can now read separate debug-info files based on a build ID

Recent GDB (at least on a vanilla Debian box) looks for debug information in

  /usr/lib/debug/.build-id/nn/nnnnnnn

where nn/nnnnnn is the build-id of the stripped ELF binary. This is
documented here:

  https://sourceware.org/gdb/onlinedocs/gdb/Separate-Debug-Files.html

This was not working in perf because we didn't read the build id until
AFTER we searched for the separate debug information file. This patch
reads the build ID and THEN does the search.

Signed-off-by: Dima Kogan <dima@secretsauce.net>
Link: http://lkml.kernel.org/r/87si6pfwz4.fsf@secretsauce.net
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>