scripts/fuzz_opt.py can not be invoked from scripts/

[zapashcanon]

Hi,

Running:

$ scripts/fuzz_opt.py

will work just fine, but:

$ cd scripts && ./fuzz_opt.py

will not because it expects the bin/wasm-opt file to be in the current directory.

[kripken]

I think you can use --binaryen-bin to set the bin dir, so it can find the tools if they aren't under ./bin/.