Fix arm disassambler with specifying armv8.1a architecture

If no architecture is defined for the disassambler command then it uses
the architecture of the target. In case of arm it will be "arm" what is
treated as the oldest arm version by the LLVM disassambler causing a lot
of invalid opcode in the output.

This change forces the use of "armv8.1a" (the newest arm architecture) if
no sub architecure was specified (either by the user or by the target) to
disassamble all instruction.

Differential revision: http://reviews.llvm.org/D12553

llvm-svn: 246648