WIP: SDK: Add gcc build-dependencies
The Proton SDK Docker image, which is used to build Proton, needs to build a newer gcc cross-compiler targeting the Windows ABI (currently using mingw-w64). We already have runtime libraries corresponding to these; if we add their development files, that's one less thing for the Proton SDK to arrange.
Part of T28539.
Resolves: #10 (closed)
Dropped from !116 (closed) for now because @rbernon says Proton isn't actually currently using soldier as the base environment to build its gcc (although perhaps it should be).