[mips] Add support for .cpload.

Summary:
This directive is used for setting up $gp in the beginning of a function.
It expands to three instructions if PIC is enabled:
lui   $gp, %hi(_gp_disp)
addui $gp, $gp, %lo(_gp_disp)
addu  $gp, $gp, $reg

_gp_disp is a special symbol that the linker sets to the distance between
the lui instruction and the context pointer (_gp).

Reviewers: dsanders

Reviewed By: dsanders

Differential Revision: http://reviews.llvm.org/D3480

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@207637 91177308-0d34-0410-b5e6-96231b3b80d8