Support cargo-driven wasm-bindgen linking - #27179

…ction (#27208)

This unifies wasm-bindgen output generation under a single
`-sWASM_BINDGEN` path, building on the wasm-bindgen side change in
wasm-bindgen/wasm-bindgen#5210. It supersedes #27179.

Previously `-sWASM_BINDGEN` was effectively a staticlib-only flow where
emcc had to discover and own the export set itself. With
wasm-bindgen/wasm-bindgen#5210, wasm-bindgen hoists its clean exported
API (functions, classes, enums) into top-level library symbols that
self-register as public exports via the `__export`/`__force` symbol
attributes (#27436, wasm-bindgen/wasm-bindgen#5253) in its
`--js-library`. The user-facing API now comes from wasm-bindgen's own
library in every flow, so emscripten no longer needs to guess or own
exports — it just surfaces what wasm-bindgen registered. That lets one
`-sWASM_BINDGEN` path serve both whether cargo/rustc drives the link
(bin crate, emcc as the linker, rustc supplies `-sEXPORTED_FUNCTIONS`)
or emcc drives it (staticlib, exports discovered locally).

The model for a public export is the union across both systems:
wasm-bindgen's self-registered API, plus emscripten's own
`EMSCRIPTEN_KEEPALIVE` exports — minus wasm-bindgen's internal expansion
glue, which must not spill into the public surface.

What's implemented:

* `-sWASM_BINDGEN` is a no-op unless the linker inputs carry
wasm-bindgen's `__wasm_bindgen_emscripten_marker` custom section
(emitted by the wasm-bindgen crate). The check runs on the inputs before
the link (`llvm-objdump --section-headers`, so archive members are
covered), so there is a single link and no speculative exports. When the
marker is absent wasm-bindgen is never invoked and need not be
installed, so cargo/rustc can pass the flag unconditionally via
`-Clink-arg` without affecting ordinary builds.
* The exports the wasm-bindgen expansion reaches by name — the supplied
`EXPORTED_FUNCTIONS` (method shims, the `__wbindgen_*` runtime, the
marker, `main`) plus anything its expansion adds — are captured as
internal glue and kept off every export layer: the ESM wrapper
(`user_requested_exports`), the factory `Module` attachment
(`EXPORTED_FUNCTIONS`, via `should_export`), and the keepalive pass in
`finalize_wasm`. `main` still runs automatically on init; `_main` isn't
surfaced.
* A genuine `EMSCRIPTEN_KEEPALIVE` C/C++ export is **not** in that
internal set and remains surfaced, so a hand-written native export
composes with wasm-bindgen's API in the same module.
* Placeholder symbols wasm-bindgen consumes (`__wbindgen_describe*`,
`__externref_*`, ...) are stripped so they aren't reported as undefined
exports.
* nm-based export discovery only runs when no driver supplied
`EXPORTED_FUNCTIONS`; the rustc-driven link already lists them exactly.
* Imported JS is wired through: `library_bindgen.extern-pre.js` is fed
as extern-pre-js and the `snippets/` dir is copied next to the output so
relative imports resolve.
* Under `WASM_ESM_INTEGRATION`, `wasmExports` is provided via a
namespace import of the wasm so wasm-bindgen's by-name export access
works. This is gated behind `WASM_BINDGEN` so plain ESM integration
keeps per-symbol named imports and their tree-shakability. (The wrapper
re-export of self-registered JS library symbols itself landed in
#27436.)

Both output modes then expose only the clean API (e.g. a `Greeter`
class).

Testing:

* End-to-end test parameterized over the ESM (`-sWASM_ESM_INTEGRATION`)
and factory (`-sMODULARIZE -sEXPORT_ES6`) output modes, built via `cargo
build` with `-sWASM_BINDGEN` passed as a link arg, asserting the clean
`Greeter` API works, `main` runs on init, and none of the raw wasm
exports leak.
* A no-marker test asserting `-sWASM_BINDGEN` is a no-op for an ordinary
`hello_world.c` build (doesn't require wasm-bindgen installed).
* The staticlib integration test is extended to assert an
`EMSCRIPTEN_KEEPALIVE` native export survives alongside the wasm-bindgen
API (fails under a blanket export wipe).
* CI installs a pinned `wasm-bindgen-cli` alongside rust so the flow is
always exercised.

Not in scope (follow-up): preserving human-supplied `EXPORTED_FUNCTIONS`
through wasm-bindgen linkage (the rustc-supplied set is currently
indistinguishable from glue, so explicit exports aren't surfaced).