This adds a wrapper around perf_event_open, exported constants and
two structs:
 - perf_event_attr, used to configure the syscall
 - perf_event_mmap_page to coordinate the kernel's mmap ring buffer

Change-Id: Ife7ded8344de9e3eb74441bd51ab9a04c81b4965
Reviewed-on: https://go-review.googlesource.com/105756Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>