Auto merge of #148291 - purplesyringa:move-throw-to-unwind, r=bjorn3

Move wasm `throw` intrinsic back to `unwind`

Fixes rust-lang/rust#148246, less invasive than the previously proposed rust-lang/rust#148269. Removes the publicly visible unstable intrinsic tracked in rust-lang/rust#122465 since it's not clear how to export it in a sound manner.

r? `@bjorn3`

---

rustc assumes that regular `extern "Rust"` functions unwind only if the `unwind` panic runtime is linked. `throw` was annotated as such, but unwound unconditionally. This could cause UB when a crate built with `-C panic=abort` called `throw` from `core` built with `-C panic=unwind`, since no terminator was added to handle the panic arising from calling an allegedly non-unwinding `extern "Rust"` function.

rustc was taught to recognize this condition since rust-lang/rust#144225 and prevented such linkage, but this caused regressions in
rust-lang/rust#148246, since this meant that Emscripten projects could not be built with `-C panic=abort` without recompiling std.

The most straightforward solution would be to move `throw` into the `panic_unwind` crate, so that it's only compiled if the panic runtime is guaranteed to be `unwind`, but this is messy due to our architecture. Instead, move it into `unwind::wasm`, which is only compiled for bare-metal targets that default to `panic = "abort"`, rendering the issue moot.

Author: bors