[mach-o] fix test case to work with latest llvm-objdump output

Take opporunity to clean up test to only run llvm-objdump once now that
llvm-objdump can disassemble mixed thumb and arm code.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@218429 91177308-0d34-0410-b5e6-96231b3b80d8