[hwasan] Madvise away thread aux data

Summary:
Release memory pages for thread data (allocator cache, stack allocations
ring buffer, etc) when a thread exits. We can not simply munmap them
because this memory is custom allocated within a limited address range,
and it needs to stay "reserved".

This change alters thread storage layout by putting the ring buffer
before Thread instead of after it. This makes it possible to find the
start of the thread aux allocation given only the Thread pointer.

Reviewers: kcc, pcc

Subscribers: kubamracek, llvm-commits

Differential Revision: https://reviews.llvm.org/D56621

llvm-svn: 352151