Auto merge of #39490 - RReverser:em-linker, r=alexcrichton
Add Emscripten-specific linker
Emscripten claims to accept most GNU linker options, but in fact most of `-Wl,...` are useless for it and instead it requires some additional special options which are easier to handle in a separate trait.
Currently added:
- `export_symbols`: works on executables as special Emscripten case since staticlibs/dylibs aren't compiled to JS, while exports are required to be accessible from JS.
Fixes #39171.
- `optimize` - translates Rust's optimization level to Emscripten optimization level (whether passed via `-C opt-level=...` or `-O...`).
Fixes #36899.
- `debuginfo` - translates debug info; Emscripten has 5 debug levels while Rust has 3, so chose to translate `-C debuginfo=1` to `-g3` (preserves whitespace, variable and function names for easy debugging).
Fixes #36901.
- `no_default_libraries` - tells Emscripten to exclude `memcpy` and co.
TODO (in future PR): dynamic linking via `SIDE_MODULE` / `MAIN_MODULE` mechanism.