Add subspace to Compiler-Explorer / Godbolt

[danakj]

Sometimes it would be very helpful to debug something on compiler-explorer, but I have to copy-paste 100 files into the tool. We can add the library to compiler-explorer? This is esp helpful for things that only repro on one compiler, and for reducing compiler bugs.

https://github.com/compiler-explorer/compiler-explorer/blob/main/docs/AddingALibrary.md