build: Probe LLVM symbol versions without llvm::Any
LLVM 24 no longer exports llvm::Any::TypeId, leaving the system LLVM
symbol-version probe empty and producing an invalid export map.
Read the default version of an LLVM export instead, without assuming
Julia's symbol-version prefix.
Assisted-by: Claude Code (Fable 5.1)
Assisted-by: Codex (gpt-6-astra)