Re-enable wasm32-unknown-emscripten target.

[Alexhuszagh]

Uses a trick described in rust-lang/rust/issues#85821#issuecomment-969369677. Note that this still allows it to run, despite there seemingly being no entry point: this has no effect on the resulting code.

Related to #609. Partially fixes #839.

[Alexhuszagh]

Note that asmjs-unknown-emscripten cannot be re-enabled, due to issues described in the updated CI action and in #839. I've tested it against stdexcept in a C++ target, which seemingly was the issue before, and it works fine. The file compiles, but when run properly complains that Rust cannot catch foreign exceptions:

#include <iostream>
#include <stdexcept>

extern "C" {
  void hello() {
    std::cout << "Hello, world!" << std::endl;
     throw std::invalid_argument( "received negative value" );
  }
}

[Alexhuszagh]

bors try --target wasm32-unknown-emscripten

[bors]

try

Build succeeded:

• conclusion

[Emilgardis]

should be added to readme too, but otherwise, nice catch!

[Alexhuszagh]

I've updated the README and also updated the extract target info for this, since I also forgot to do this for solaris. Feel free to merge this whenever ready.

[Emilgardis]

bors r0

[Emilgardis]

bors r+

[bors]

Build succeeded:

• conclusion

[Jules-Bertholet]

The upstream Rust issue has since been resolved (rust-lang/rust#8582 (comment)), and Emscripten has also made a change to support older Rust versions again (emscripten-core/emscripten#17222). The emscripten fix has made it to the latest release (https://hub.docker.com/layers/emsdk/emscripten/emsdk/3.1.14/images/sha256-4c3e0a0dac61430b719e82118ae9b2c7480902a2713267e80fa296d39f7ab921). So this PR should probably be reverted in favor of a bump to the emscripten version used in the Dockerfile