[mips] Implement .cplocal directive

This directive forces to use the alternate register for context pointer.
For example, this code:
  .cplocal $4
  jal foo
expands to:
  ld    $25, %call16(foo)($4)
  jalr  $25

Differential Revision: https://reviews.llvm.org/D64743

llvm-svn: 366300