Rollup merge of #148536 - folkertdev:cmse-async-const-fn, r=davidtwco

cmse: add test for `async` and `const` functions

tracking issue: rust-lang/rust#81391
tracking issue: rust-lang/rust#75835

Some additional tests that seemed useful while working on the RFC text.

`async` functions are disallowed (because `-> impl Trait` is not supported).

`const` entry functions are allowed, `nonsecure-call` does not make sense, because this abi can only be used on function pointers, which cannot be evaluated during constant evaluation.

The async test is in the `c-variadic.rs` file because it has the minicore-compatible machinery for defining an async function. Splitting that logic out (like `minisimd.rs`) turns out to be complicated because the async stuff relies on types defined by minicore.

r? `````@davidtwco`````

Author: Zalathar