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

[Anutosh Bhat via llvm-commits]

anutosh491 wrote:

cc @sbc100 

I see the following variable responsible for building shared libs.

https://github.com/llvm/llvm-project/blob/1b9805c14dbaa3af39cf3d9e5a88d3b150c69dc8/llvm/docs/CMake.rst#L314

I think that might be enough to get stuff working.
Would it make sense to enforce this in case of emscripten. Something like 
```
if(EMSCRIPTEN)
    BUILD_SHARED_LIBS=OFF
endif()
```

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