[llvm-objdump] Set --print-imm-hex by default.

This was previously attempted in 2016 by colinl's D18770, but LLD tests
were missed, which caused the change to be reverted.

Setting --print-imm-hex by default brings llvm-objdump's behavior closer
in line with objdump, and it makes it easier to read addresses and
alignment from the disassembly. It may make non-address immediates
harder to interpret, but it still seems the better default, barring more
context-sensitive base selection logic.

Differential Revision: https://reviews.llvm.org/D136972