Add a bare metal plugin (for "none" triples)

[jakepetroules]

This is just going in SWBUniversalPlatform for now for simplicity / bringup, and should maybe move into a new SWBNonePlatformPlugin later on.

Closes #726

[jakepetroules]

@swift-ci test

[jakepetroules]

Please keep in mind that a none triple does not mean Swift Embedded, or that you even have Swift at all. Do not make assumptions that you're in such an environment when using this SDK/platform. It should generalize to any "none" triple, including a plain C environment.

[jakepetroules]

@swift-ci test

[dschaefer2]

Please keep in mind that a none triple does not mean Swift Embedded, or that you even have Swift at all. Do not make assumptions that you're in such an environment when using this SDK/platform. It should generalize to any "none" triple, including a plain C environment.

Correct. From the SwiftPM perspective, these platforms are generally accompanied by a toolset.json file which contains most importantly a --sysroot argument pointing to the C runtime to be used in the build. It also contains architecture specific options for the C and Swift compilers and the linker. And often, but not always, will enable the Embedded feature, at least for microcontroller platforms.

That said, the extraCLIOptions from the toolset get tacked into the extraArgs and passed to the SwiftBuild request along with the other swift build CLI args.

Longer term, we probably should look at how to do these platforms as proper SDKs with an SDKSettings.json instead of the toolset.json/sysroot to unify how we present platforms to SwiftBuild.

[jakepetroules]

Longer term, we probably should look at how to do these platforms as proper SDKs with an SDKSettings.json instead of the toolset.json/sysroot to unify how we present platforms to SwiftBuild.

Maybe. It's not clear to me yet whether SDKSettings should be the canonical format used everywhere vs one remaining specific to Apple platforms. Its schema is fairly ad-hoc and maybe not the best fit for something more general purpose.

[jakepetroules]

@swift-ci test

[jakepetroules]

@swift-ci test macOS

[jakepetroules]

@swift-ci test

[jakepetroules]

@swift-ci test macOS

[jakepetroules]

@swift-ci test