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

[Anutosh Bhat via llvm-commits]

anutosh491 wrote:

Hey @serge-sans-paille 

One final request here would be if you could add the milestone (**LLVM 20.X Release** I think)

So that this is picked up in the next release (20.0.2) and we wouldn't need to use the patch then !

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