This hopefully fixes Trac #14264 after a long road of breakage,
- D3977 refactored the backend to use mkAsmTempLabel (through BlockId.blockLbl) to produce labels for BlockIds as this is what the NCG did. This fixes Trac #14221.
- This in turn broke the LLVM and unregisterised backends (Trac #14264)
- D4014 attempted to fix this by removing an assertion, fixing the LLVM backend but leaving the unregisterised configuration broken
- b3ae47caf2f23cfd2c22c29dbfca646493ffe469 restored the assertion but still didn't fix the underlying problem: that AsmTempLabels aren't supported by the unregisterised backend.
This hopefully fixes this by using proper EntryLabels to represent block labels.