ARM IAS: support .short and .hword

This adds support for the .short and its alias .hword for adding literal values
into the object file.  This is similar to the .word directive, however, rather
than inserting a value of 4 bytes, adds a 2-byte value.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@201968 91177308-0d34-0410-b5e6-96231b3b80d8