[llvm-objdump] Switch between ARM/Thumb based on mapping symbols.

[llvm-objdump] Switch between ARM/Thumb based on mapping symbols.

The ARMDisassembler changes allow changing between ARM and Thumb mode
based on the MCSubtargetInfo, rather than the Target, which simplifies
the other changes a bit.

I'm not really happy with adding more target-specific logic to
tools/llvm-objdump/, but there isn't any easy way around it: the logic
in question specifically applies to disassembling an object file, and
that code simply isn't located in lib/Target, at least at the moment.

Differential Revision: https://reviews.llvm.org/D60927

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@363903 91177308-0d34-0410-b5e6-96231b3b80d8