[JITLink] Add support for MachO/x86-64 UNSIGNED relocs with length=2.

MachO/x86-64 UNSIGNED relocs are almost always 64-bit (length=3), but UNSIGNED
relocs of length=2 are allowed if the target resides in the low 32-bits. This
patch adds support for such relocations in JITLink (previously they would have
triggered an unsupported relocation error).

llvm-svn: 367764