[ELF][MIPS] Handle R_MIPS_HI16/LO16 relocations against _gp_disp symbol

The `_gp_disp` is a magic symbol designates offset between start of
function and gp pointer into GOT. Only `R_MIPS_HI16` and `R_MIPS_LO16`
relocations are permitted with `_gp_disp`. The patch adds the `_gp_disp`
as an ignored symbol and adjusts symbol value before call the `relocateOne`
for `R_MIPS_HI16/LO16` relocations.

Differential Revision: http://reviews.llvm.org/D15480

llvm-svn: 255768