[llvm-readobj][MachO] Fix section type printing

Summary:
Currently, llvm-readobj mistakenly decodes section type as section attribute.

This patch fixes the bug and affected tests.

Reviewers: JDevlieghere, jhenderson, rupprecht, alexshap, echristo

Reviewed By: jhenderson, rupprecht, alexshap, echristo

Subscribers: javed.absar, rupprecht, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D66075

llvm-svn: 368974