Two intrinsics call C functions on the Rust stack

[brson]

rust_intrinsic_recv and rust_intrinsic_task_sleep both call functions in rustrt, which means they are calling functions on the Rust stack without doing the stack check. rust_intrinsic_recv can be rewritten as a regular builtin, and rust_intrinsic_task_sleep needs to do everything inline.

[nikomatsakis]

It occurs to me that the need for a stack check may be a reason to have wrappers even around the intrinsics...otherwise, the generated C++ code does not do the stack check. Good that I just put some effort into tearing the wrappers out. :)

[brson]

I think it will do the check without wrappers. We're translating the ll asm as part of our rust crate, so as long as everything gets inlined when we generate intrinsics.ll, when rustc finally compiles it, we'll be inserting our rust prologues into the C functions.

[brson]

rust_intrinsic_cast does it too

[brson]

cast and recv no long call C functions. yield still does, but I suspect it is small enough to run in the red zone