Changes from v4:
* Add Drew's Reviewed-by to first patch.
* Explain use of 32-bit cycle count values in AArch32.
* Zero-initialize pmu_data struct before use in check_cycles_increase and
check_cpi. While the insistence on not using memset is entirely my own vanity,
I blame the funny syntax on https://gcc.gnu.org/bugzilla/show_bug.cgi?id=53119
(adds extra set of braces) and checkpatch (adds spaces).
* Improve formatting of inline assembly and better explain why so much code
must be inline assembly.
