Add a lock ivar to the Platform so that multiple Targets
trying to populate the list of trap handler names at
the same time don't conflict with one another.
<rdar://problem/17011969> 


git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@209563 91177308-0d34-0410-b5e6-96231b3b80d8