Allow for optional content in a report file

[klingaard]

Would be really handy to use the same report definition file for multiple configurations that might or might not have the same stats. Today the report definition causes the framework to error:

Unable to parse expression: "cpu.icache.stats.il1_cache_misses" within context: top in report file "report.yaml" for the following reason: 
SPARTA was unable to find a tree node that matched this name.

Here's what the report.yaml looks like:

name: "Summary ICache Report"
content:
  cpu.icache.stats.il1_cache_misses: ICache Misses

This works for architecture A which has the icache at the cpu level, but not architecture B which has it maybe inside fetch.

Suggested "fix":

name: "Summary ICache Report"
optional_content:
  # Works for arch A
  cpu.icache.stats.il1_cache_misses: ICache Misses
optional_content:
  # Works for arch B
  cpu.fetch.icache.stats.il1_cache_misses: ICache Misses

[klingaard]

Implementation example:

name: "Summary Performance Report"
style:
  decimal_places: 2
content:
  cycles : Total cycles consumed
simple-arch-content:
  cpu.core*.rob.stats.total_number_retired : Total Instructions Retired
other-arch-content:
  cpu.core*.rob.stats.ipc : IPC

The report mechanism looks for the keyword <arch name>-arch-content where <arch name> is the value supplied on the command line via --arch <arch name> flag