i#6426 sched stats: Add histogram of instrs per switch

[derekbruening]

Adds a very simple histogram with fixed bin sizes recording the distribution of instructions per context switch in the schedule_stats tool.

Updates two tests to confirm the histogram is produced.

This feature is structured to allow replacing this simple histogram with a more sophisticated version in subclasses.

Issue: #6426

[derekbruening]

x32 failure is type_is_instr #3320