[ARM] Fix llvm-objdump disassembly of armv7m object files.

Apparently, the features were getting mixed up, so we'd try to
disassemble in ARM mode. Fix sub-architecture detection to compute the
correct triple if we're detecting it automatically, so the user doesn't
need to pass --triple=thumb etc.

It's possible we should be somehow tying the "+thumb-mode" target
feature more directly to Tag_CPU_arch_profile? But this seems to work
reasonably well, anyway.

While I'm here, fix up the other llvm-objdump tests that were explicitly
specifying an ARM triple; that shouldn't be necessary.

Differential Revision: https://reviews.llvm.org/D106912