asmjs-unknown-emscripten target, disabled

[Emilgardis]

This target is failing.

https://github.com/cross-rs/cross/runs/5084819916?check_suite_focus=true

https://gist.github.com/Emilgardis/3283deb43cdab4fd6f21877022921736

Disabled for now.

[Emilgardis]

#609 (comment)

[Alexhuszagh]

Partially fixed as of #840, depends on the upstream patch for rust-lang/rust#98216.