utils/llvm-lit: Generate "llvm-lit.py" rather than "llvm-lit" on Win32 hosts to let llvm-lit.py --use-processes work.

llvm-lit needs suffix.py for multiprocess to find a main module.
check-llvm: Ask llvm-config about assertion mode, instead of llc.

Add --assertion-mode to llvm-config. It emits ON or OFF according to NDEBUG.