ARM IAS: add support for the .pool directive

The .pool directive is an alias for the .ltorg directive used to create a
literal pool.  Simply treat .pool as if .ltorg was passed.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@197787 91177308-0d34-0410-b5e6-96231b3b80d8