[mips][mips64r6] Add Relocations R_MIPS_PCHI16, R_MIPS_PCLO16 
Differential Revision: http://reviews.llvm.org/D3860


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@209659 91177308-0d34-0410-b5e6-96231b3b80d8