JavaScriptKit fails to link with Embeded Swift in Swift 6 language mode

[MaxDesiatov]

Reproduction

1. Clone Example/EmbeddedConcurrency introduced in Fix Embedded, bump 6.3 Swift toolchain in test.yml swiftwasm/JavaScriptKit#702
2. Modify Package.swift to removed .swiftLanguageMode(.v5) so it defaults to Swift 6 from swift-tools-version:6.0
3. Build with latest development snapshot

TOOLCHAINS=org.swift.64202603091a swift build \
    --package-path /tmp/EmbeddedConcurrency-repro \
    --swift-sdk swift-DEVELOPMENT-SNAPSHOT-2026-03-09-a_wasm-embedded

Expected behavior

Everything links and builds fine

Environment

Reproducible on both macOS and Linux