llvm-mc: Add option for prefering hex format disassembly.

Previously there was a separate mode entirely (--hdis vs.
--disassemble). It makes a bit more sense for the immediate printing
style to be a flag for --disassmeble rather than an entirely different
thing.
ARM: honor hex immediate formatting for ldr/str i12 offsets.

Previously we would always print the offset as decimal, regardless of
the formatting requested. Now we use the formatImm() helper so the value
is printed as the client (LLDB in the motivating example) requested.

Before:
ldr.w r8, [sp, #180] @ always

After:
ldr.w r8, [sp, #0xb4] @ when printing hex immediates
ldr.w r8, [sp, #0180] @ when printing decimal immediates

rdar://17237103