[mips] Make the MipsAsmParser capable of knowing whether PIC mode is enabled or not.

Summary:
This information is needed to decide whether we do the PIC-only JAL expansions or not. It's also needed for an upcoming patch which implements the .cprestore assembler directive (which can only be used effectively in PIC mode).

By making this information available to the MipsAsmParser, we will know when to insert the instructions mandated by the .cprestore assembler directive and we will be able to give some useful warnings when we encounter a potential misuse of this directive.

Patch by Toma Tabacu

Reviewers: dsanders, seanbruno

Subscribers: brooks, seanbruno, rafael, llvm-commits

Differential Revision: http://reviews.llvm.org/D5626

llvm-svn: 245291