ARM IAS: support .inst directive

This adds support for the .inst directive.  This is an ARM specific directive to
indicate an instruction encoded as a constant expression.  The major difference
between .word, .short, or .byte and .inst is that the latter will be
disassembled as an instruction since it does not get flagged as data.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@197657 91177308-0d34-0410-b5e6-96231b3b80d8