[lld] Special R_MIPS_HI16 / R_MIPS_LO16 calculation for _gp_disp symbol

Separated from the "[lld] R_MIPS_HI16 / R_MIPS_LO16 calculation" thread.

Ah, I didn't realize that relocateOne doesn't take a symbol.

I'm not sure what's the simplest way, but you might want to add a new
parameter to relocateOne.