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).

llvm-svn: 296429

lld/ELF/InputFiles.cpp

@@ -744,6 +744,7 @@ static uint8_t getBitcodeMachineKind(MemoryBufferRef MB) {
   case Triple::aarch64:
     return EM_AARCH64;
   case Triple::arm:
+  case Triple::thumb:
     return EM_ARM;
   case Triple::mips:
   case Triple::mipsel: