Bump download-ci-llvm-stamp for loong support

This was missed in rust-lang#96971 and
resulted in the LLVM we cache in CI being different from the one built
locally. We didn't catch it because nothing tested the loong support.

Author: jyn514

src/bootstrap/download-ci-llvm-stamp

@@ -1,4 +1,4 @@
 Change this file to make users of the `download-ci-llvm` configuration download
 a new version of LLVM from CI, even if the LLVM submodule hasn’t changed.
 
-Last change is for: https://github.com/rust-lang/rust/pull/109373
+Last change is for: https://github.com/rust-lang/rust/pull/96971