perf tools fixes for v6.6: 2nd batch

- Fix regression in reading scale and unit files from sysfs for PMU
  events, so that we can use that info to pretty print instead of
  printing raw numbers:

  # perf stat -e power/energy-ram/,power/energy-gpu/ sleep 2

   Performance counter stats for 'system wide':

              1.64 Joules power/energy-ram/
              0.20 Joules power/energy-gpu/

       2.001228914 seconds time elapsed
  #
  # grep -m1 "model name" /proc/cpuinfo
  model name	: Intel(R) Core(TM) i7-8650U CPU @ 1.90GHz
  #

- The small llvm.cpp file used to check if the llvm devel files are present was
  incorrectly deleted when removing the BPF event in 'perf trace', put it back
  as it is also used by tools/bpf/bpftool, that uses llvm routines to do
  disassembly of BPF object files.

- Fix use of addr_location__exit() in dlfilter__object_code(), making sure that
  it is only used to pair a previous addr_location__init() call.

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