Enable `_Concurrency` for Embedded Swift with WASI (#79292)
WASI with Embedded Swift provides WASI-libc and libc++ headers necessary to build the `_Concurrency` module for Wasm. We now add `wasm32-unknown-wasip1-wasm` triple to `EMBEDDED_STDLIB_TARGET_TRIPLES` when `SWIFT_WASI_SYSROOT_PATH` is set, which builds the necessary stdlib slice.
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Co-authored-by: Yuta Saito <kateinoigakukun@gmail.com>
