[llvm-objdump] Correctly distinguish between the MachO upper/lower16 relocations

All other code in MachODump.cpp uses the same comparison,
((r_length & 0x1) == 1), for distinguishing between the two,
while the code in llvm-objdump.cpp seemed to be incorrect.

Differential Revision: https://reviews.llvm.org/D35240

llvm-svn: 307882