adding functions for easy access to cycle counters

["Blair Sadewitz" <blair.sadewitz%gmail.com@localhost>]

Hello,

Does anyone have an opinion on adding functions in
src/common/lib/libc/atomic for reading CPU cycle counters?  I thought
of this while working on a JACK audio package:  I've already converted
it to use the atomic ops in userland.  If I had an MI function to read
the cycle counter, e.g. rtdsc on x86, then it could build easily on
any platform NetBSD-current supports.

These functions don't _have_ to be located with the atomic ops, I just
figured it was a logical place to put them.

Regards,

--Blair