Keeps track of instructions (including operands) that are executed,
to see if there are any hotspots that could be optimized or fastpaths
that should be added.
Also adds a mode where CPU profiler data is periodically output, to
make it easier to get at these instruction counts during startup.
Result of running IWYU (https://include-what-you-use.org/) and
applying most of the suggestions about unncessary includes and
forward declarations.
Was motivated by observing that <thread> was being included in
ppcopcodes.cpp even though it was unused (found while researching
the use of threads), but seems generally good to help with build
times and correctness.
