[mips] Add support for '.option pic2'.

The directive '.option pic2' enables PIC from assembly source.
At the moment none of the macros/directives check the PIC bit
but that's going to be fixed relatively soon. For example, the
expansion of macros like 'la' depend on the relocation model.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@204803 91177308-0d34-0410-b5e6-96231b3b80d8