ARM: by default don't set the Thumb bit on MachO relocated values.

ARM: by default don't set the Thumb bit on MachO relocated values.

Its existence is largely historical, apparently we tried to make ARM object
files look maybe-almost-possibly runnable by putting our best guess at the
actual value into relocated locations. Of course, the real linker then comes
along and can completely change things.

But it should only be there for word-sized and movw/movt relocations. It can't
be encoded in branch relocations, and I've seen it mess up validity
calculations twice in the last couple of weeks so the default is clearly problematic.

llvm-svn: 279773