Avoid using MCSymbolData on the asm streamer.

Only the object streamers need to track if a symbol should be marked thumb or
not. This ports the ELF case. The COFF case is not ported since it is currently
not working for some other reason (I will report a bug).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@207366 91177308-0d34-0410-b5e6-96231b3b80d8