Object: rename ARMV7 to ARMNT

The official specifications state the name to be ARMNT (as per the Microsoft
Portable Executable and Common Object Format Specification v8.3).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@203530 91177308-0d34-0410-b5e6-96231b3b80d8