Add an example of PEvent generation

[klingaard]

This SHA is an example of how to generate PEvents (Performance Events) typically used for correlation
efforts between a perf model and hardware.

PEvents are a more "compact" form of the pipeout collection pairs, allowing a modeler to specify more direct items
for collection. Pipeout pairs can be more elaborate/detailed. However, with some clever coding, there can be
reuse -- it's not done here.

To generate performance events, use the command line parameter --pevents <filename> <pevent1,pevent2,...|all>.

Examples:

# All pevents
./olympia traces/dhry_riscv.zstf --pevents retire.out all -i100

# Just RETIRE pevent
./olympia traces/dhry_riscv.zstf --pevents retire.out RETIRE -i100

# RETIRE and COMPLETE pevent
./olympia traces/dhry_riscv.zstf --pevents retire.out COMPLETE,RETIRE -i100

While I was at it, I removed the generated annotation for pipeline collection for LoadStore/MemoryAccess classes -- uses NVP which generates faster, as well as creates smaller pipeouts.

[furuame]

Looks good to me! You may also add a test for it.

[klingaard]

Looks good to me! You may also add a test for it.

Done! Zhen keeping me honest. 😛