[winasan] Unpoison the stack in NtTerminateThread

In long-running builds we've seen some ASan complaints during thread creation that we suspect are due to leftover poisoning from previous threads whose stacks occupied that memory. This patch adds a hook that unpoisons the stack just before the NtTerminateThread syscall.

Differential Revision: https://reviews.llvm.org/D52091

llvm-svn: 342652