[libunwind] Fix comment about phase (NFC) (#206660)

We switched to use `_UA_SEARCH_PHASE` in
https://github.com/emscripten-core/emscripten/pull/17991, but the
comment above was not fixed, and it was upstreamed that way later.

GitOrigin-RevId: 443d933cef6ef8dc5179ff8ffe9feb133a265920