[mach-o] Fix missing { }

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@217411 91177308-0d34-0410-b5e6-96231b3b80d8
[mach-o] add testing coverage for -single_module

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@217412 91177308-0d34-0410-b5e6-96231b3b80d8