[mips][mips64r6] Add relocations R_MIPS_PC21_S2, R_MIPS_PC26_S2 
Differential Revision: http://reviews.llvm.org/D3824


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@209655 91177308-0d34-0410-b5e6-96231b3b80d8