ARM big.LITTLE support

[Vince Weaver]

There have been requests about whether PAPI supports heterogeneous processors (such as ARM big.LITTLE).

I'm not sure how difficult it would be. It might involve having 2 sets of presets valid at once.

This would require libpfm4 support, see: https://groups.google.com/a/icl.utk.edu/forum/#!searchin/perfapi-devel/big$20little/perfapi-devel/IrAj926zfBo/PvrMLEJhAAAJ