[mips64] Emit correct addend for some PC-relative relocations

So far, LLVM has not emitted correct addend for N64 and N32 ABI. This patch
fixes that. It also removes fixup from MCJIT for R_MIPS_PC16 relocation.

Patch by Vladimir Radosavljevic.

Differential Revision: http://reviews.llvm.org/D10565

llvm-svn: 240404