don't allow AsmTempLabel in UNREG mode (Trac Trac #14264)
AsmTempLabel is really a label that describes
label in assembly output (or equivalent like LLVM IR).
Unregisterised build does not handle it correctly.
This change does not fix UNREG build failure in
Ticket Trac #14264 but reverts back to panic:
Signed-off-by: Sergei Trofimovich <email@example.com>