Add perf_reader_consume

Add perf_reader_consume

In order to read 'remainder' events from perf buffers w/ 'wakeup_events
> 1', an API to force reading from a program's perf buffers regardless
of whether their fds are in "ready to read" state is necessary. This
commit introduces such an API, perf_reader_consume, modeled after
libbpf's perf_buffer__consume.

Future work will refactor bcc to use libbpf's perf buffer
support as much as possible instead of duplicating functionality. For
now, since #3801 was commited let's add this piece of missing
functionality.