[mips] Implement 'la' macro in PIC mode for O32.

Summary:
N32 support will follow in a later patch since the symbol version of 'la'
incorrectly believes N32 to have 64-bit pointers and rejects it early.

This fixes the three incorrectly expanded 'la' macros found in bionic.

Reviewers: sdardis

Subscribers: dsanders, llvm-commits, sdardis

Differential Revision: http://reviews.llvm.org/D20820

llvm-svn: 271644