wasm32-wasip1-threads: Correct llvm target name

[kolayne]

Intends to fix #131007, a problem with cross-compiling Rust+C++ to wasm32-wasip1-threads: clang will be told to compile for --target=wasm32-wasi, which it interprets as wasm32-wasip1, which is not compatible with wasm32-wasip1-threads.

I wonder if there might be any compatibility issue or something depending on the old incorrect name...

[rustbot]

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[mu001999]

r? alexcrichton

[alexcrichton]

Apologies but I no longer maintain this wasm target, so I'd prefer it if someone else took a look at this.

r? compiler