Basic pmu implmentation + callgate fixes

[evanstella]

Summary of this Pull Request (PR)

Basic PMU support and callgate fixes

Intent for your PR

Choose one (Mandatory):

• [ ] This PR is for a code-review and is intended to get feedback, but not to be pulled yet.
• [ ] This PR is mature, and ready to be integrated into the repo.

Reviewers (Mandatory):

(Specify @<github.com username(s)> of the reviewers. Ex: @user1, @user2)

Code Quality

As part of this pull request, I've considered the following:

Style:

• [ ] Comments adhere to the Style Guide (SG)
• [ ] Spacing adhere's to the SG
• [ ] Naming adhere's to the SG
• [ ] All other aspects of the SG are adhered to, or exceptions are justified in this pull request
• [ ] I have run the auto formatter on my code before submitting this PR (see doc/auto_formatter.md for instructions)

Code Craftsmanship:

• [ ] I've made an attempt to remove all redundant code
• [ ] I've considered ways in which my changes might impact existing code, and cleaned it up
• [ ] I've formatted the code in an effort to make it easier to read (proper error handling, function use, etc...)
• [ ] I've commented appropriately where code is tricky
• [ ] I agree that there is no "throw-away" code, and that code in this PR is of high quality

Testing

I've tested the code using the following test programs (provide list here):

• [ ] micro_booter
• [ ] unit_pingpong
• [ ] unit_schedtests
• [ ] ...(add others here)

[gparmer]

Thanks again for this! Always amazing work.