[ELF][MIPS] Calculate combined addend for R_MIPS_GOT16 against local symbol

R_MIPS_GOT16 relocation against local symbol requires index of a local
GOT entry which contains page address corresponds to sum of the symbol
address and addend. The addend in that case is calculated using addends
from the R_MIPS_GOT16 and paired R_MIPS_LO16 relocations.

Differential Revision: http://reviews.llvm.org/D17610

llvm-svn: 261930