Fix llvm-objdump so it picks a good CPU based for Mach-O files

for CPU_SUBTYPE_ARM_V7S and CPU_SUBTYPE_ARM_V7K.

For these two cpusubtypes they should default to a cortex-a7 CPU
to give proper disassembly without a -mcpu= flag.

rdar://27431703

llvm-svn: 292993