Do not add uwtable attribute by default for MachO targets.

r217178 changed clang to add function attribute uwtable by default on
Win64, which caused the __eh_frame section to be emitted by default.
This commit restores the previous behavior for MachO targets.

rdar://problem/25282627


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@268589 91177308-0d34-0410-b5e6-96231b3b80d8