[NativeProcessLinux] Reduce the number of casts
NPL used to be peppered with casts of the NativeThreadProtocol objects into NativeThreadLinux. I
move these closer to the source where we obtain these objects. This way, the rest of the code can
assume we are working with the correct type of objects.
Reviewers: ovyalov, tberghammer
Differential Revision: http://reviews.llvm.org/D12187