tweaks for wasm-opt running logic

[dgryski]

1) if -work was passed don't overwrite original. .wasm binary 2) if -x was passed print out wasm-opt command line.

[jameysharp]

I asked Damian for this because it helps with troubleshooting bugs introduced by wasm-opt if we can check what the wasm looked like before that. For example, I was just now able to use this PR to verify that llvm-dwarfdump --verify finds no errors in the DWARF data before wasm-opt, but finds a bunch of errors after wasm-opt, for this program:

package main
import "os"
func main() {
        os.Lstat("some-filename")
}