[PowerPC] Support .llong and fix .word

This adds support for the .llong PowerPC-specifc assembler directive.
In doing so, I notices that .word is currently incorrect: it is
supposed to define a 2-byte data element, not a 4-byte one.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@185911 91177308-0d34-0410-b5e6-96231b3b80d8