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.