Add arc_prefetch script.

[brad-lewis]

See DLPX-68394

Here is sample output:
02/12/20 - 17:21:40 UTC

prefetch hit count : 21
prefetch miss count : 17
prefetch read count : 1
microseconds arc read latency
value range count ------------- Distribution -------------
[16, 32) 9 |@@@@@@@@@@@@@@@@@@
[32, 64) 6 |@@@@@@@@@@@@
[64, 128) 3 |@@@@@

microseconds prefetch read latency
value range count ------------- Distribution -------------
[16M, 32M) 1 |@

                            avg latency(us)

arc read latency 39
prefetch read latency 34658844

[brad-lewis]

I updated BCCHelper with a couple things. One was to add the BCCMapIndex and the key CUSTOM index type. The purpose of this was to allow ints to be used for the map keys in the C code with strings being substituted for the display. The second BCCPoolCompare using the approach Seb suggested of getting the spa_t pointer first and using that as an input into the main tracing script. Both of these can be used to simplify the existing scripts. Once the final form is reviewed and approved I can do the refactoring.