oprofile 0.9.8: fix compilation on powerpc

The oprofile build was broken on powerpc since version 0.9.8.
This was detected in several autobuilds, like
http://autobuild.buildroot.net/results/6f6c02d18495907d50fcdfc6003ac20d493c55fe/



Thomas Petazzoni had some fixes pending in his own tree, and this patch is
partially based on this work (credits to him). Here is an overview:
- I took over (and fixed) the oprofile.mk changes, except for the powerpc-
  specific part. For powerpc, there is a new dependency to libpfm4.
- I reimported those Yocto patches that were specific to the ppc build
  issues, but left out the other ones. Those can be added in separate
  commits.

[Peter: simplify libpfm4 check]
Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Tested-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>