[Mips] Reduce code duplication. Join relocation handling functions which
perform calculations for R_MIPS_GOT16 and R_MIPS_CALL16 relocations.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@209594 91177308-0d34-0410-b5e6-96231b3b80d8