Add Triple::thumb to getBitcodeMachineKind
We really need to find a way to get this info from a single point of
truth in the LLVM backend, but it seems that the EM_* constants are
buried deep inside the constructors of the MCAsmBackend's.
For now, just fill in entries as we run into cases. AFAIK these mappings
are largely immutable, so we get a 75% discount on the technical debt
(code is duplicated, but little chance of divergence).