[ELF][MIPS] Add MipsGotSection to handle MIPS GOT

MIPS GOT handling is very different from other targets so it is better
to keep the code in the separatre section class MipsGotSection. This
patch introduces the new section and moves all MIPS specific code from
GotSection to the new class. I did not rename fields and methods in the
MipsGotSection class to reduce the diff and plan to do that by the
separate commit.

Differential revision: https://reviews.llvm.org/D26733

llvm-svn: 287150