perf_counter tools: Cover PLT symbols too

PLT, the Program Linking Table, is used with the dynamic linker to
allow PIC code in executables and shared objects to figure out
where functions are in other shared objects.

It is one of the sources of unknown/unresolved symbols - this patch
does what binutils figures out when you ask it to disassembly.
(objdump -S)

Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Corey Ashford <cjashfor@linux.vnet.ibm.com>
Cc: Marcelo Tosatti <mtosatti@redhat.com>
Cc: John Kacur <jkacur@redhat.com>
Cc: Stephane Eranian <eranian@googlemail.com>
LKML-Reference: <new-submission>
Signed-off-by: Ingo Molnar <mingo@elte.hu>