fuzz_opt.py fixes (#2836)
fuzz_opt.py fixes (#2836)

Use --emit-target-features and --detect-features.

Rename Asyncify handler temp files (I happened to notice they overwrote other files, which was annoying.

Fixes #2831
WebAssembly/binaryen,WebAssembly/binaryen,WebAssembly/binaryen,WebAssembly/binaryen,WebAssembly/binaryen