build: Repair the build on WASI platform

[kateinoigakukun]

Fixes package-nized build for WASI of _CoreFoundation.

$ swift --version
Swift version 6.0-dev (LLVM b7b175a4a1263ea, Swift 07eb52a80dee5a1)
Target: x86_64-unknown-linux-gnu
$ swift experimental-sdk install https://github.com/swiftwasm/swift/releases/download/swift-wasm-DEVELOPMENT-SNAPSHOT-2024-03-29-a/swift-wasm-DEVELOPMENT-SNAPSHOT-2024-03-29-a-ubuntu22.04_x86_64.artifactbundle.zip
$ swift build --experimental-swift-sdk DEVELOPMENT-SNAPSHOT-2024-03-29-a-wasm --target _CoreFoundation

The Foundation product needs macro with cross-compilation support in SwiftPM, so it's still not working yet.