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

llvm-svn: 210701