i#3544 RISC-V, part 6: Add extension stubs

[semihalf-kardach-stanislaw]

Add definitions required for extensions to compile for RISC-V.

NOTE: This code is not validated and mostly contains stubs as the main point was to achieve compilation and estimate the effort required for the port. Some of the trivial logic has been implemented though.

Issue: #3544

Signed-off-by: Stanislaw Kardach [email protected]

[derekbruening]

I can take this one; please request a review from me when ready.

[semihalf-kardach-stanislaw]

Thanks! I'll be off for the next 8 days, so I'll address any comments when I get back.

[derekbruening]

(Please re-request review when ready)

[semihalf-kardach-stanislaw]

Seems that azure.archive.ubuntu.com is down which causes CI to fail at apt-get. I'll re-try tomorrow morning (CET).