libpmc: Add support for IBS qualifiers

Add support to libpmc for parsing the IBS qualifiers and computing the
ctl register value as a function of the qualifiers and the sample rate.
This includes all of the flags available up to AMD Zen 5. Along side
these user facing changes I included the documentation for AMD IBS.

Reviewed by: mhorne
Sponsored by: Netflix
Pull Request: https://github.com/freebsd/freebsd-src/pull/2081