SCons: Add stack_size and default_pthread_stack_size options to Web target

[nikitalita]

This adds the following compile options to web builds with emscripten:

• stack_size: the stack size for the main thread (this cannot be adjusted at runtime), defaults to 5MB
• default_pthread_stack_size: the default stack size for any pthreads spawned, defaults to 2MB

The reason for these options being added is because versions of emscripten prior to 3.1.27 had the above default values. emscripten 3.1.27 adjusted the defaults of these down to 64KiB. This ended up breaking a lot of things in Godot; the editor would simply crash due to a stack overflow upon load.

We may want to adjust the default values for these downwards for non-editor builds, but I figured I should just set it back to where it was for now.

[nikitalita]

@Faless can you please take a look at this? This is still an issue and there's no easy way to pass these flags into emscripten otherwise.

For verification purposes, you can see the default values for 3.1.26 vs. 3.1.27-current (3.1.50) here:

• default stack size in 3.1.26 is 5MB
• default pthread stack size in 3.1.26 is 2MB
• 3.1.27 default stack size is 64KB
• 3.1.27 pthread stack size is 64KB
• current (3.1.50) default stack size is 64KB
• current (3.1.50) pthread stack size is 64KB (it's set to 0, which makes it point to the STACK_SIZE argument, which is 64KB by default)

[akien-mga]

Sorry for the delay. Seems like we overlooked this, or the fact that it's fixing a critical build issue. It's described in the OP but was overlooked as this looked more like adding a new feature, than fixing a bug. For next time I may suggest opening a bug report so it would have been clear to us (and notably me as buildsystem maintainer) that we have a critical compatibility issue with Emscripten >= 3.1.27.

We debugged this issue from another bug report #85564 and fixed it yesterday: #86036.

So this fix is no longer needed technically. But the added configurability makes sense to me, so I think it can be rebased to make the new link flags added in #86036 use the options you're adding here.

[Faless]

This PR should be rebased on #86036 and updated to change the flags values in place.
Right now merging it will result in some flags being set two times / overridden by the other change.

[akien-mga]

Rebased on the latest master myself to solve the double definition of those flags.

[akien-mga]

Thanks!

[akien-mga]

Cherry-picked for 3.6.

[YuriSizov]

Cherry-picked for 4.2.2.