[mips] Add the following MIPS options that control gp-relative addressing of
small data items: -mgpopt, -mlocal-sdata, -mextern-sdata. Implement gp-relative
addressing for constants.

Differential Revision: http://reviews.llvm.org/D4903


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@221450 91177308-0d34-0410-b5e6-96231b3b80d8