[mips] Add assembler support for the "la $reg,symbol" pseudo-instruction.

Summary:
This pseudo-instruction allows the programmer to load an address from a symbolic expression into a register.

Patch by David Chisnall.
His work was sponsored by: DARPA, AFRL

I've made some minor changes to the original, such as improving the formatting and adding some comments, and I've also added a test case.

Reviewers: dsanders

Reviewed By: dsanders

Differential Revision: http://reviews.llvm.org/D4808