Bump Binaryen version to 121 (#23207)

WebAssembly/binaryen#7153 updated Binaryen
version to 121, and we updated it in Emscripten to 120 to make the CI
pass (#23197). Now that
https://chromium-review.googlesource.com/c/emscripten-releases/+/6103023
has landed, I think we can update it to 121.

tools/building.py

@@ -39,7 +39,7 @@
 
 #  Building
 binaryen_checked = False
-EXPECTED_BINARYEN_VERSION = 120
+EXPECTED_BINARYEN_VERSION = 121
 
 _is_ar_cache: Dict[str, bool] = {}
 # the exports the user requested