[ELF] Support 128-bit bitmask in oneof(RelExpr)

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)`.

Reviewed By: arichardson, peter.smith

Differential Revision: https://reviews.llvm.org/D112385