Add riscv as a zephyr target

[jdavidberger]

I'm working on a FPGA based SOC that uses zephyr and this snippet is required to enable thread / other zephyr OS support for riscv targets

[JanMatCodasip]

Hi @jdavidberger, thanks for your PR.

As this repository is stale and probably going to be archived, you may want to submit your change into the upstream OpenOCD instead: review.openocd.org.

Few hints, in case you do:

• Hint 1: Your PR will have higher chance of acceptance in the upstream if you describe the steps how your change can be tested. This way reviewers will have a chance of re-testing your changes. E.g. what platform to use (a specific board board? spike simulator?), how to build a simple Zephyr-based program, ...

• Hint 2: There are some trivial whitespace issues (mixture of spaces and tabs).