benchmark

Filter performance counter names, not invalidate all (#1554)

* Filter performance counter names, not invalidate all

Currently, the performance counters are validated while they
are being created and one failure returns NoCounters(), ie it
effecitvely invalidates all the counters.

I would like to propose a new behavior: filter instead. If an
invalid name is added to the counter list, or if that particular
counter is not supported on this platform, that counter is dropped
from the list and an error messages is created, while all the
other counters remain active.

This will give testers a peace of mind that if one mistake is made
or if something is changed or removed from libpfm, their entire
test will not be invalidated. This feature gives more tolerance
with respect to versioning.

Another positive is that testers can now input a superset of all
desired counters for all platforms they support and just let
Benchmark drop all those that are not supported, although it will
create quite a lot of noise down the line, in which case perhaps
we should drop silently or make a consolidated, single error line
but this was not implemented in this change set.

* Removed unused helper type.