Rust ABI is not the C ABI

[brson]

Having a well-defined ABI is probably important eventually. Right now it's nice to think the Rust ABI is just the C ABI but it is actually not since LLVM's cdecl does not implement the complete platform-specific rules for the C ABI. If we wanted to actually define the Rust ABI as the C ABI then we would need to run our code generation through the same code paths as our native function bindings.

[brson]

cc #1835

[nikomatsakis]

Do we necessarily want to specify the Rust ABI precisely? I like the
idea that the Rust ABI is undefined but is distinct from the C abi. I
guess we might want to specify it for forwards binary compatibility at
some point, but I think that's not something we have to do now (I
think we'd want to come up with a precise list of the sorts of changes
that warrant recompilation; right now it's kind of "anything at all",
and changing Rust compiler versions definitely counts).

[steveklabnik]

Given that this would be an RFC-worthy topic these days, closing in favor of rust-lang/rfcs#600