Revisit the default of `-fvisibility=hidden`

;TLDR; visibility is currently only used by emscripten and emscripten uses a default of -fvisibility=default.

A long while ago we choose to make -fvisibility=hidden the default for the WebAssembly backend in llvm. IIRC the rational was that we would want to avoid exporting a lot of C/C++ symbols by default.

However, the only time the wasm-ld linker actually exports things based on their visibility is when -shared is used (or in the static linking case when --export-dynamic is passed). These flags are not currently used outside of emscripten. For example, with wasi-sdk users are currently things via on of 3 different methods:

1. Explicit --export on the command line (e.g. --export=foo)
2. --export-all on the command line. This is even worse that exporting non-hidden symbols since it exports everything (do we even what to support this going forward?)
3. export_name attribute in the source code.

In emscripten, the visibility attribute is honored when -shared/-pie is used, and we recently switch from using --export-all to --export-dynamic + -fvisibility=default which results in fewer exports (for example --export-all exports all libc-internal symbols , even those marked as hidden, whereas --export-dynamic + -fvisibility=default does not). See https://github.com/emscripten-core/emscripten/pull/15413.

Start with the wasm-ld linker behavior described here, then compare how emscripten and wasi-sdk use -fvisibility=hidden/default, -shared/-pie, --export-dynamic, --export-all, and export_name. Review the linked emscripten change for context. Done requires an agreed convention for the default and its supported export mechanisms, followed by coordinated implementation and validation.