wasm: add recover support

Add unwinding and recover support for wasm using WebAssembly exception
handling. This still has a few gotchas:

  * Many WASI systems don't support exception handling yet.
    For example, see:
    https://github.com/bytecodealliance/wasmtime/issues/2049
  * Asyncify doesn't support wasm exception handling:
    https://github.com/WebAssembly/binaryen/issues/4470
    This means it's not possible to use goroutines together with
    panic/recover.
  * The current way that exceptions are implemented pretend to be C++
    exceptions, but work slightly differently. If C++ code is called
    (for example through CGo) that raises an exception, that exception
    will be eaten by TinyGo and not be propagated. This is fixable, it
    just hasn't been implemented (because we don't actually support C++
    right now).

I hope that these issues will be resolved over time. At least for now,
people who need `recover()` have a way to use it.

transform/gc.go

@@ -177,7 +177,7 @@ func MakeGCStackSlots(mod llvm.Module) bool {
 				// now.
 				// With more analysis, it should be possible to optimize a
 				// significant chunk of these away.
-			case llvm.Call, llvm.Load, llvm.IntToPtr:
+			case llvm.Call, llvm.Invoke, llvm.Load, llvm.IntToPtr:
 				// These create new values so must be stored locally. But
 				// perhaps some of these can be fused when they actually refer
 				// to the same value.