Assigning an opcode to `features.supported` instruction

[dtig]

What is the intended opcode for the features.supported instruction?

Options:

• At the end of the opcode space for the MVP opcodes (i.e. 0xc5)
• As a pre-fixed post-MVP opcode at the end of the opcode space (we can bikeshed which one based on the other proposals in flight)

[tlively]

I don't expect feature detection to require a large number of opcodes, so I think it would be fine to use an un-prefixed opode. @rossberg, do you have an opinion on this? (assuming for now that feature detection would require just 2 new opcodes)

[titzer]

I think we should preserve opcode space for instructions that may end up being common in the future, or to use as prefix pages. So I would support a relatively long (i.e. prefixed) encoding for features.supported.