[ARM] Create objdump subtarget from build attrs

[ARM] Create objdump subtarget from build attrs

Enable an ELFObjectFile to read the its arm build attributes to
produce a target triple with a specific ARM architecture.
llvm-objdump now uses this functionality to automatically produce
a more accurate target.

Differential Revision: https://reviews.llvm.org/D28769

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@292366 91177308-0d34-0410-b5e6-96231b3b80d8