oprofile: needs libpfm4 on powerpc64(le)

oprofile: needs libpfm4 on powerpc64(le)

oprofile already needed libpfm4 for PowerPC, but in fact it also needs
it for PowerPC64 and PowerPC64le, so this commit adds a
BR2_PACKAGE_OPROFILE_NEEDS_LIBPFM4 hidden option and uses it to decide
if libpfm4 is needed or not.

Fixes:

  http://autobuild.buildroot.net/results/d19caeb445b7040cbd2e4ff62b7db9986e84b6ab/ (powerpc64le)
  http://autobuild.buildroot.net/results/66dd4f7ff635915a506f96d1d8115ac48cca0c6c/ (powerpc64)

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>