Move wasm throw intrinsic back to unwind

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: purplesyringa

crates/core_arch/src/wasm32/mod.rs

@@ -173,32 +173,3 @@ pub fn f64_nearest(a: f64) -> f64 {
 pub fn f64_sqrt(a: f64) -> f64 {
     crate::intrinsics::sqrtf64(a)
 }
-
-unsafe extern "C-unwind" {
-    #[link_name = "llvm.wasm.throw"]
-    fn wasm_throw(tag: i32, ptr: *mut u8) -> !;
-}
-
-/// Generates the [`throw`] instruction from the [exception-handling proposal] for WASM.
-///
-/// This function is unlikely to be stabilized until codegen backends have better support.
-///
-/// [`throw`]: https://webassembly.github.io/exception-handling/core/syntax/instructions.html#syntax-instr-control
-/// [exception-handling proposal]: https://github.com/WebAssembly/exception-handling
-#[cfg_attr(test, assert_instr(throw, TAG = 0, ptr = core::ptr::null_mut()))]
-#[inline]
-#[unstable(feature = "wasm_exception_handling_intrinsics", issue = "122465")]
-// FIXME: Since this instruction unwinds, `core` built with `-C panic=unwind`
-//        cannot be linked with `-C panic=abort` programs. But that's not
-//        entirely supported anyway, because runtimes without EH support won't
-//        be able to handle `try` blocks in `-C panic=unwind` crates either.
-//        We ship `-C panic=abort` `core`, so this doesn't affect users
-//        directly. Resolving this will likely require patching out both `try`
-//        and `throw` instructions, at which point we can look into whitelisting
-//        this function in the compiler to allow linking.
-//        See https://github.com/rust-lang/rust/issues/118168.
-#[allow(ffi_unwind_calls)]
-pub unsafe fn throw<const TAG: i32>(ptr: *mut u8) -> ! {
-    static_assert!(TAG == 0); // LLVM only supports tag 0 == C++ right now.
-    wasm_throw(TAG, ptr)
-}