[mach-o] Fix arm interworking with movw/movt

In some cases the address of a function will be materialized with a movw/movt
pair.  If the function is a thumb function, the low bit needs to be set on
the movw immediate value.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@214277 91177308-0d34-0410-b5e6-96231b3b80d8