x86-64: Ignore r_addend for R_X86_64_GLOB_DAT/R_X86_64_JUMP_SLOT

According to x86-64 psABI, r_addend should be ignored for R_X86_64_GLOB_DAT
and R_X86_64_JUMP_SLOT.  Since linkers always set their r_addends to 0, we
can ignore their r_addends.

Reviewed-by: Fangrui Song <maskray@google.com>