[spec] Preserve host function arguments on divergence

[sanguineman]

Summary

Previously, the reduction rule for host function divergence dropped the call arguments val^n from the resulting configuration.

According to the WebAssembly specificaiton, the post-conditions for executing host functions must match those in the execution rule for invoking host functions. However, a host function that takes one or more arguments, one reduction step by Step/call_ref-hostfunc-div removes val^n and leaves only (REF.FUNC_ADDR a) (CALL_REF yy). Neither host-function execution rule can then apply because the required operands are missing.

Fix

The updated rule preserves the complete configuration, making the divergence case as exact self-loop as intended.

[rossberg]

Good catch, thanks!