add c_variadic_experimental_arch feature

[folkertdev]

tracking issue: #155973

Based on https://hackmd.io/pIbUgMQuQcGaibJcinOcEw#Stabilize-c-variadic-function-definitions-rust155697, we'll gate niche targets where we don't control the implementation of va_arg, the ABI is unclear, or in general where we're not confident stabilizing the implementation.

[joshtriplett]

This looks reasonable to me, based on the discussions in the lang meeting. We can always un-gate any of these targets in the future, when a maintainer of the target takes a close look and makes sure the tests pass.

I think it might be useful to point people to an invocation of ./x test that will capture all the relevant tests. For instance, does ./x test variadic capture all the tests needed to feel confident that a target implements C variadics correctly?

r=me with such a comment added next to the match arm for the unstable targets. (Not next to the match arm for RISC-V ILP32E, that one is unstable for a different reason.)

@bors delegate+

[rust-bors]

✌️ @folkertdev, you can now approve this pull request!

If @joshtriplett told you to "r=me" after making some further change, then please make that change and post @bors r=joshtriplett.

View changes since this delegation.