[mips] Initial implementation of -mabicalls/-mno-abicalls.

This patch implements the main rules for -mno-abicalls such as reserving $gp,
and emitting the correct .option directive.

Patch by Matheus Almeida and Toma Tabacu

Differential Revision: http://reviews.llvm.org/D4231

llvm-svn: 215194