Use --swift-sdk selector as SDK name for Wasm platform

[kateinoigakukun]

Use --swift-sdk selector as SDK name for Wasm platform

Motivation:

The expected SDK name for Wasm platform support in SwiftBuild is always Swift SDK id, not "webassembly", so passing always passing "webassembly" makes SwiftBuild unable to resolve a particular Swift SDK.

Close #9364

Modifications:

Propagate --swift-sdk value as SDK name to SwiftBuild for Wasm targets.

Result:

With this change and swiftlang/swift-build#998, we can build a simple hello world package with --build-system swiftbuild with Swift SDK for Wasm.

$ swift build --swift-sdk DEVELOPMENT-SNAPSHOT-2025-12-19-a-wasm32-unknown-wasip1 --build-system swiftbuild
Building for debugging...
[0/100] Using on-disk description
[100/100]
Build complete! (0.91 secs)