Slightly refactor things for llvm-objdump and the -macho option so it can be used...

options other than just -disassemble so that universal files can be used with other
options combined with -arch options.

No functional change to existing options and use.  One test case added for the
additional functionality with a universal file an a -arch option.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@225383 91177308-0d34-0410-b5e6-96231b3b80d8