cmm: Use LocalBlockLabel instead of AsmTempLabel to represent blocks
blockLbl was originally changed in 8b007abb to use mkTempAsmLabel to fix an inconsistency resulting in #14221. However, this breaks the C code generator, which doesn't support AsmTempLabels (#14454). Instead let's try going the other direction: use a new CLabel variety, LocalBlockLabel. Then we can teach the C code generator to deal with these as well.