Use assert in addition to abort for WASM_UNREACHABLE

[tlively]

This yields more useful information when something goes wrong.

[kripken]

An assert would only happen if not NDEBUG, so this changes the behavior in a release build without asserts (note that our CI only builds release+asserts currently, so we are missing possible compiler failures from a change like this, due to not having a noreturn function there).

How about using an assert when it would have an effect, and __builtin_unreachable otherwise? Do you know what LLVM does for unreachable?

[kripken]

lgtm with that fix

[kripken]

We should enclose both in { } as otherwise bad stuff would happen with this:

if (..) WASM_UNREACHABLE();