Tighten/correct/sync may_leave checks for component model intrinsics

[alexcrichton]

Right now I think more-or-less all intrinsics check for may_leave, but reading over CanonicalABI.md there are some that do not check this:

• resource.rep
• context.get
• context.set
• backpressure.*
• thread.index
• thread.available-parallelism