Taken from Chih-Mao Chen's D100835.
RelExpr has 64 bits now and needs the extension to support new members
(`R_PLT_GOTPLT` for `R_X86_64_PLTOFF64` support).
Note: RelExpr needs to have at least a member >=64 to prevent
-Wtautological-constant-out-of-range-compare for `if (expr >= 64)`.