Un-break the big-endian buildbots


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@221919 91177308-0d34-0410-b5e6-96231b3b80d8
R600: Fix assert on empty function

If a function is just an unreachable, this would hit a
"this is not a MachO target" assertion because of setting
HasSubsectionViaSymbols.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@221920 91177308-0d34-0410-b5e6-96231b3b80d8