[mips] Add the following MIPS options that control gp-relative addressing of

[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

llvm-svn: 221450