Exceptions: support Pulley

[cfallin]

The exception-handling support as implemented in #11326 does not yet support Pulley. It is almost there, but not quite:

• Pulley's use of Cranelift for code generation means that we do get exception metadata alongside Pulley bytecode, and a module that uses exceptions can be compiled;
• The support for stack-walking of Pulley stacks means that the exception unwinder can find handlers in Pulley code;
• However, the raise libcall implementation that Pulley uses does not account for UnwindState::UnwindToWasm. This means that we'll reach host-to-Wasm entry instead, with an unexpected unwind state, and panic.

In this comment I outline an alternative approach: we should

• invoke the real raise libcall, just as we do other libcalls, from Pulley;
• when we enter unwind, and either wasmtime_longjmp() or wasmtime_unwinder::resume_to_exception_handler(), we instead check the store's executor and
  • on native code, do as we do today
  • on Pulley, update the Pulley interpreter state appropriately

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