[CP-beta][skwasm] Implement Wasm worker futex semaphores - #196127

Replaces Emscripten's no-op `-sWASM_WORKERS=1` POSIX semaphore stubs
(`sem_init`, `sem_destroy`, `sem_post`, `sem_wait`, `sem_trywait`) with
a WebAssembly atomic futex implementation in multi-threaded Skwasm and
Web Impeller (`wimp`) builds.

### Rationale

Under Emscripten's `-sWASM_WORKERS=1` runtime without `-pthread`
(`skwasm_heavy` / `wimp_heavy`), `libc-ww.a` compiles
`system/lib/pthread/library_pthread_stub.c`, which stubs `sem_init`,
`sem_wait`, `sem_trywait`, `sem_post`, and `sem_destroy` (as well as
`emscripten_futex_wait` / `emscripten_futex_wake`) to `return 0`.

In Skia (`third_party/skia/src/core/SkSemaphore.cpp`), `SkSemaphore`
uses POSIX `sem_t` on `__EMSCRIPTEN__` as the contended slow-path
backing for `SkMutex` and `SkSharedMutex`. Because `sem_wait` is a
no-op, any contended acquisition on FreeType's global `f_t_mutex()`
(`third_party/skia/src/ports/SkFontHost_FreeType.cpp:221-226`) or
`SkTypeface_FreeType::fC2GCacheMutex`
(`third_party/skia/src/ports/SkTypeface_FreeType.h:87`) returns
immediately when the browser main UI thread performs `Paragraph::layout`
/ `MakeTextFrameFromTextBlobSkia` concurrently with the background
render worker rasterizing glyphs from the shared `FT_Library` /
`FT_Face`, causing memory corruption and worker stalls
([flutter#193695](flutter#193695)).

### Summary of Changes

-
[`engine/src/flutter/skwasm/wasm_worker_semaphore.cc`](https://github.com/kevmoo/flutter/blob/87e66a27b6b/engine/src/flutter/skwasm/wasm_worker_semaphore.cc)
&
[`engine/src/flutter/skwasm/BUILD.gn`](https://github.com/kevmoo/flutter/blob/87e66a27b6b/engine/src/flutter/skwasm/BUILD.gn):
Override `sem_init`, `sem_destroy`, `sem_post`, `sem_wait`, and
`sem_trywait` via linker `-Wl,--wrap=sem_*` in multi-threaded
`skwasm_variant` (`#if defined(__EMSCRIPTEN_WASM_WORKERS__)`) builds:
- Stores the atomic counter inline in `sem_t`'s 16-byte `__val` buffer
(`cache/sysroot/include/semaphore.h:17-19`), requiring zero heap
allocation or teardown in `sem_destroy`.
- Background `DedicatedWorker` threads
(`emscripten_wasm_worker_self_id() != 0`, cached in a `thread_local
bool` to avoid a Wasm-to-JS FFI call per contended wait) sleep via
`emscripten_atomic_wait_u32` and wake via `emscripten_atomic_notify`.
- The browser main thread (`emscripten_wasm_worker_self_id() == 0`,
where `memory.atomic.wait32` / `Atomics.wait` is disallowed by the
WebAssembly Threads spec because `[[CanBlock]] == false`) uses an atomic
compare-exchange spin-acquire loop.

### Verification

- Built `wasm_release:skwasm/skwasm_heavy.js` and
`wasm_release:skwasm/wimp_heavy.js` and verified `20/20` multi-threaded
text-grid runs (`grid@100` and `grid@1000`) complete without `FT_Face`
memory corruption or worker stalls in Chrome 154.

Fixes flutter#193695

## Pre-launch Checklist

- [x] I read the [Contributor Guide] and followed the process outlined
there for submitting PRs.
- [x] I read the [AI contribution guidelines] and understand my
responsibilities, or I am not using AI tools.
- [x] I read the [Tree Hygiene] wiki page, which explains my
responsibilities.
- [x] I read and followed the [Flutter Style Guide], including [Features
we expect every widget to implement].
- [x] I signed the [CLA].
- [x] I listed at least one issue that this PR fixes in the description
above.
- [x] I updated/added relevant in-code documentation (doc comments with
`///`).
- [x] If this PR introduces a new feature or capability, I created and
linked a website documentation issue or PR in [flutter/website] (or
verified none is needed).
- [x] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [x] I followed the [breaking change policy] and added [Data Driven
Fixes] where supported.
- [x] All existing and new tests are passing.

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