Cranelift: implement an "unwinder" crate and exception throws in filetests.

[cfallin]

This commit introduces the next major piece of machinery (after the previously-landed try_call support) that we will eventually use to implement Wasm exceptions in Wasmtime. In particular, it implements a generic unwinder as a new crate that supports (i) walking a stack produced by Cranelift code, (ii) serializing Cranelift exception metadata to compact tables (in a way very similar to address maps in Wasmtime, so they will be mappable directly from disk), (iii) using these serialized tables to find handlers during a stack-walk, and (iv) jumping to handlers (i.e., actually unwinding). This crate is currently used in the filetests runner, and will next be used in Wasmtime.

The commit first performs code-motion: it moves stack-walking code from Wasmtime to cranelift-unwinder. This itself has no functional effect, but isolates the code that understands contiguous sequences of Cranelift frames ("activations") from that which is specific to Wasmtime's activation delimiters and metadata.

It then implements a compact exception-table format. This format uses the object crate's mechanisms for directly referencing in-memory arrays of little-endian u32s in a way that will allow us to find handlers when mapping exception metadata directly from an ELF section in a .cwasm (for example). The format consists of four sorted u32 arrays in a way that allows us to look up a callsite first, then search its sorted array of handler offsets by tags.

It next implements the actual unwind control flow: it contains an assembly stub for each supported architecture that transfers control to a PC, SP, and FP value "up the stack", with payload values placed in the payload registers we have defined per our exception ABI in Cranelift.

Finally, it puts these pieces together in the filetest runner. Note that the runtest does a lot "by hand": we don't have entry and exit trampolines as we do in Wasmtime, so the filetest contains three functions, with the middle one invoking the "throw hostcall" and entry and exit trampolines around it grabbing the appropriate entry/exit FPs and exit PC. The dance to call back to host code is also somewhat delicate, as we haven't done this before. The JITModule's linking + relocation support does not seem sufficient to properly define a symbol, so instead we scan for func_addr instructions referencing a well-known name (__cranelift_throw) and replace them with iconsts with the function address at runtime, baking it in. This is somewhat ugly, but it works. All of these filetest-specific details will be handled much more nicely in the Wasmtime version of this functionality, as we have proper abstractions for entry/exit trampolines and hostcalls.

[bjorn3]

By the way why is the unwind info format defined in the cranelift-unwinder crate? It is Wasmtime specific and I would assume that Winch and Pulley will reuse the same format.

[fitzgen]

r=me with things we discussed in the cranelift meeting

[cfallin]

Wow, CI really does not like this PR -- network error here downloading CMake.