Tracking issue for wasm32-emscripten ui test failures

These are the tests that are failing after #131705:

• abi/numbers-arithmetic/return-float.rs -- adding -Cpanic=abort fixes the failure. The upstream bug report has a C++ reproducer. More reason it would be nice to resolve #112195.
• async-await/issue-60709.rs -- Emscripten bug that occurs when -lc and -Oz are passed to the linker, can be reproduced with

  echo "fn main(){}" > a.rs
  rustc -Copt-level=z a.rs -o a.js --target=wasm32-unknown-emscripten
  

• backtrace/dylib-dep.rs -- "Not supported", xfailed in https://github.com/rust-lang/rust/pull/131776
• backtrace/line-tables-only.rs -- "Not supported", xfailed in https://github.com/rust-lang/rust/pull/131776
• no_std/no-std-unwind-binary.rs -- compiler says error: lang item required, but not found: eh_catch_typeinfo
• structs-enums/enum-rec/issue-17431-6.rs -- One of the two compiler errors is missing
• test-attrs/test-passed.rs

Start by reviewing the listed wasm32-unknown-emscripten UI tests, especially abi/numbers-arithmetic/return-float.rs and no_std/no-std-unwind-binary.rs, then reproduce the provided rustc command for the Emscripten linker failure. Done means the unchecked failures are fixed or appropriately accounted for, while the listed unsupported cases remain covered by their existing xfails.