Fix a bug in llvm-objdump when disassembling using the wrong default CPU

in the second slice of a Mach-O universal file.

The code in llvm-objdump in in DisassembleMachO() was getting the default
CPU then incorrectly setting into the global variable used for the -mcpu option
if that was not set.  This caused a second call to DisassembleMachO() to use
the wrong default CPU when disassembling the next slice in a Mach-O universal
file.  And would result in bad disassembly and an error message about an
recognized processor for the target:

% llvm-objdump -d -m -arch all  fat.macho-armv7s-arm64 
fat.macho-armv7s-arm64 (architecture armv7s):
(__TEXT,__text) section
armv7:
       0:	60 47 	bx	r12
fat.macho-armv7s-arm64 (architecture arm64):
'cortex-a7' is not a recognized processor for this target (ignoring processor)
'cortex-a7' is not a recognized processor for this target (ignoring processor)
(__TEXT,__text) section
___multc3:
       0:		.long	0x1e620810

rdar://34439149

llvm-svn: 313921