Make one mutex for the lldb_private::Platform class that can be used to protect with...

Make one mutex for the lldb_private::Platform class that can be used to protect with modifying member variables. This mutex is designed to be used for simple modifications, so the lock should be taken, modify the member variable and released. We need to make sure this isn't used with any code that cause code to rely or reenter on another thread.

Partial fix for: <rdar://problem/19575304>

llvm-svn: 227855