[llvm] Disable shared libraries when building with emscripten (PR #118262)

[Fangrui Song via llvm-commits]

MaskRay wrote:

`BUILD_SHARED_LIBS` defaults to `OFF`. `ON` builds shared objects like libLLVMFoo.so libLLVMBar.so. I don't think we should accept

```
if(EMSCRIPTEN)
    BUILD_SHARED_LIBS=OFF
endif()
```

You could report an error if EMSCRIPTEN is incomaptible with BUILD_SHARED_LIBS=on.

https://github.com/llvm/llvm-project/pull/118262