Support `-Cpanic=unwind` on WASI targets

This commit is some minor updates/restructuring in a few locations with
the end result being supporting `-Cpanic=unwind` on WASI targets. This
continues to be off-by-default insofar as WASI targets default to
`-Cpanic=abort`, meaning that actually using anything in this commit
requires `-Zbuild-std`. Specifically the changes made here are:

* The self-contained sysroot for WASI targets now contains a copy of
  `libunwind.a` from wasi-sdk, first shipped with wasi-sdk-33 (also
  updated here).
* The `unwind` crate here in this repository uses the `libunwind` module
  instead of the custom bare-metal wasm implementation of exceptions.
  This means that Rust uses the `_Unwind_*` symbols which allows it to
  interoperate with C/C++/etc.
* Wasm targets are all updated to pass the LLVM argument
  `-wasm-use-legacy-eh=false` to differ from LLVM's/clang's default of
  using the legacy exception handling proposal for WebAssembly. This has
  no effect by default because `panic=abort` is used on most targets.
  Emscripten is exempted from this as the Emscripten target is
  explicitly intended to follow LLVM's/clang's defaults.
* There's a single test in the test suite that links to the
  `panic_unwind` crate which ended up requiring `-Wexceptions` from
  Wasmtime, so the test parts were updated and Wasmtime was updated in
  CI, too.

The net result of all of this is that this should not actually affect
any WebAssembly target's default behavior. Optionally, though, WASI
programs can be built with exception handling via:

    RUSTFLAGS='-Cpanic=unwind' cargo +nightly run -Z build-std --target wasm32-wasip2

Effectively `-Zbuild-std` and `-Cpanic=unwind` is all that's necessary
to enable this support on wasm targets.

Finally, this ends up closing 154593 as well. The WASI targets are now
defined to use `-lunwind` to implement unwinding. This means that the
in-tree definition of `__cpp_exception` is no longer of concern and the
definition is always sourced externally. If Rust is linked with other
C/C++ code using WASI then these idioms are compatible with wasi-sdk,
for example, to use that as a linker. The main caveat is that when using
an external linker the `-fwasm-exceptions` argument needs to be passed
to `clang` for it to be able to find the `libunwind.a` library to link
against.

Closes 154593