objdump: Better handling of Mach-O universal binaries

Summary:
With Mach-O, there is a flag requirement discrepancy between working with
universal binaries and thin binaries. Many flags that don't require the `-macho`
flag (for example `-private-headers` and `-disassemble`) fail to work on
universal binaries unless `-macho` is given. When this happens, the error
message is unhelpful, stating:

    The file was not recognized as a valid object file.

Which can lead to confusion.

This change allows generic flags to be used on universal binaries with and
without the `-macho` flag. This means flags that can be used for thin files can
be used consistently with fat files too.

To do this, the universal binary support within `ParseInputMachO()` is extracted
into a new function. This new function is called directly from `DumpInput()`
when the input binary is universal. Additionally the `-arch` flag validation in
`ParseInputMachO()` was extracted to be reused.

Reviewers: compnerd

Reviewed By: compnerd

Subscribers: keith, llvm-commits

Differential Revision: https://reviews.llvm.org/D48702

llvm-svn: 338792