Make `--output-source-map-url` `-osu` update an existing source map section (if it exists) and otherwise add one

[mkustermann]

It seems when running wasm-opt with -ism app.unopt.wasm.map -osm app.opt.wasm.map -osu myUrl.wasm and the existing wasm file has already a source map url section, then wasm-opt will add another one - resulting in 2 source map sections.

This creates ambiguity for any consumer of the wasm file - which source map url to use or may even result in errors / not using any if there's multiple.

Could wasm-opt replace the existing section (if exists) instead of adding a new one?

(It seems like a common issue one would run into: Any compiler that emits a unoptimized wasm file may embed a relative source map url into that unoptimized wasm module. When optimizing using wasm-opt it most likely uses different filenames for the optimized wasm file and corresponding source map, therefore needing to update the source map url section)

[kripken]

Good catch! Fix in #8191

[mkustermann]

Thanks for the fast fix, @kripken !