Bevel a 2x6 long ways
Bandsaw, but still tricky.
Reliable solution would be to throw away half the board (not get two usable pieces from each board) and use a router sled to plane off the wood not used. Router sleds are an amazing tool to have in your list of options, and produce great work.
Traditional blended approach and probably good enough for framing is to cut both sides with a tablesaw as far as it reaches (which might be more than halfway for a 10&quot; belt-driven saw, and you are done) and then finish with a handsaw guided by the tablesaw kerfs to complete the cut.
If attempting this with a hand-held circular saw: A: Don't B: check your insurance coverage and have someone looking on to call 911. That is a bad cut to try and do with a hand-held circular saw, in my experienced opinion.
I also agree with the commenters that there is no way you need a full width beveled top plate. Normal rectangular top plate and (easily made) wedges under each joist will be done before you are done setting up to make the thing you don't need but think you do.