Support unwinding on panics

[bjorn3]

Cranelift doesn't yet support unwinding: bytecodealliance/wasmtime#1677

[bjorn3]

Status update

Merged

On the Cranelift side:

• Cranelift: remove block params on critical-edge blocks. bytecodealliance/wasmtime#10485 by @cfallin
• Cranelift: remove return-value instructions after calls at callsites. bytecodealliance/wasmtime#10502 by @cfallin
• Cranelift: initial try_call / try_call_indirect (exception) support. bytecodealliance/wasmtime#10510 by @cfallin adds the actual exception handling support.
• A bunch of regalloc2 PR's by @cfallin to fix issues exposed by the exception handling support.
• Some fixes for try_call bytecodealliance/wasmtime#10593 by myself
• A couple of minor local changes in the try_call_changes of my wasmtime fork. Cranelift: move exception-handler metadata into callsites. bytecodealliance/wasmtime#10609 by @cfallin

On the cg_clif side:

• ab514c9
• 9495eb5

Not yet merged

• The upstream-exception-handling branch of this repo adds code to actually codegen the cleanup path and emit unwind tables.
• https://github.com/bjorn3/eh_frame_experiments has the code to write .eh_frame LSDAs in the .gcc_except_table format.

I'm currently working on getting everything into a mergeable state. After that it will take at least a month before support can be merged on the cg_clif side as a Cranelift release with these changes need to be made.