Note newly enabled features in LLVM in ChangeLog (#21853)

llvm/llvm-project#80923 enabled multivalue and
reference-types by default. Even though this is an LLVM change, given
that we release emsdk with all LLVM/Binaryen/Emscripten versions, it
could be worth mentioning in the ChangeLog.

ChangeLog.md

@@ -27,6 +27,10 @@ See docs/process.md for more on how version tagging works.
   of pthread builds so that is generated alongside the main JavaScript file.
   See #21701. ()
 - `-sASYNCIFY=2` is setting now deprecated, use `-sJSPI` instead.
+- Due to the upstream LLVM change
+  (https://github.com/llvm/llvm-project/pull/80923), multivalue and
+  reference-types features are now enabled by default in Emscripten. This only
+  enables the language features and does not turn on the multivalue ABI.
 
 3.1.58 - 04/23/24
 -----------------