SafeStack: Use correct unsafe stack sizes

Summary:
When deallocating thread stacks, we use one thread's unsafe stack size
to deallocate another thread's unsafe stack; however, the two sizes may
differ. Record an unsafe stack's size in the thread stack linked list.

Reviewers: pcc, eugenis

Reviewed By: eugenis

Subscribers: delcypher, llvm-commits, #sanitizers, kcc

Differential Revision: https://reviews.llvm.org/D51016

llvm-svn: 340308