[clang] [llvm] Define LLVM_ABI and CLANG_ABI for __EMSCRIPTEN__ builds (PR #131578)

[Anutosh Bhat via llvm-commits]

anutosh491 wrote:

/cherry-pick [e57cd10](https://github.com/llvm/llvm-project/commit/e57cd100ca297cf81854e35cccbf703edddd4aad)

https://github.com/llvm/llvm-project/pull/131578