[llvm-objdump,ARM] Fix .byte directives dumping the wrong byte.

The clause in `dumpARMELFData` that dumps a single byte as a `.byte`
directive was printing the operand of that directive as `Bytes[0]`,
not `Bytes[Index]`. In particular, this led to the `dumpBytes` output
to its left not matching it!

Reviewed By: DavidSpickett

Differential Revision: https://reviews.llvm.org/D130360