Add support for direct rdpmc access

[jimblandy]

The perf_event_mmap_page structure permits direct access to hardware performance monitoring counters, via the index field, protected by a sequence lock. We could use this to provide extremely light-weight measurement APIs.

(I have never tried this, but @eddyb says they're going to try it out.)

[jimblandy]

Possibly relevant:
Can SeqLocks Get Along With Programming Language Memory Models?

tl;dr: SeqLock code like that shown in the rdpmc example code on the perf_event_open man page, use non-atomic speculative racing loads, which are UB in Rust. They'll need to become atomic reads, at least.