mark core::arch::wasm32::unreachable as safe
mark core::arch::wasm32::unreachable as safe

This intrinsic doesn't have any preconditions and is always safe to
call, so it can be safe.

This function is already stable, but dropping `unsafe` is a backwards
compatible change.

Note tha we already have a precedent for wasm intrinsics being safe --
wasm simd is safe.

It is relatively practically important to mark this safe --
wasm32::unreachable is directly useful in practice as more codesize
efficient `panic!()`.

rust-lang/stdarch,rust-lang/stdarch,rust-lang/stdarch,rust-lang/stdarch,rust-lang/stdarch