SCons: Default optimize to auto, fixing target/dev_build inference for Web

[akien-mga]

• Fixes Web builds are no longer built with -Os by default #94087.

This is a bit awkward because we can't have both a custom default value provided by each platform's get_opts, and still use some platform-agnostic "auto" behavior in SConstruct. So with this approach we go back to the 4.2 behavior where the web platform's get_opts simply overrides whatever is in optimize, unless it's specified on the command line.

I think it's an ok compromise for now, especially given how the web platform is sensitive to binary size. But it's a quirk worth knowing and documenting for people trying to debug web exports. (See also #91800.)

@dsnopek @Faless @adamscott Could also be considered to fix - #93476 by reintroducing the previous behavior, i.e. dev builds for web will still use -Os like in 4.2. Web builds with optimize=none will likely still be broken, but they're probably simply way too big for browsers to handle?

[akien-mga]

I'll merge this already to fix the regression for the 4.3-beta3 build I want to start now.

Reviews still welcome to fix up any potential mistake or bad design decision.

[Repiteo]

Makes sense to me; feels right in-line with how we handle architecture.

[dsnopek]

Could also be considered to fix - #93476 by reintroducing the previous behavior, i.e. dev builds for web will still use -Os like in 4.2. Web builds with optimize=none will likely still be broken, but they're probably simply way too big for browsers to handle?

Yeah, this does fix #93476 too - I'll comment and close that one in a moment.

I think an argument could be made that -Os is needed for web by default even on debug builds for the reason you give above. Since it was forced to -Os for so long previously and no one complained, maybe that's a good sign?