[mips] Add assembler support for the .cprestore directive.

Summary:
This assembler directive is used in O32 PIC to restore the current function's $gp after executing JAL's. The $gp is first stored on the stack at a user-specified offset.
It has the following format: ".cprestore 8" (where 8 is the offset).

This fixes llvm.org/PR20967.

Patch by Toma Tabacu.

Reviewers: seanbruno, tomatabacu

Subscribers: brooks, seanbruno, emaste, llvm-commits

Differential Revision: http://reviews.llvm.org/D6267

llvm-svn: 247897