tsan: mmap shadow stack

We used to mmap C++ shadow stack as part of the trace region
before ed7f3f5b ("tsan: move shadow stack into ThreadState"),
which moved the shadow stack into TLS. This started causing
timeouts and OOMs on some of our internal tests that repeatedly
create and destroy thousands of threads.
Allocate C++ shadow stack with mmap and small pages again.
This prevents the observed timeouts and OOMs.
But we now need to be more careful with interceptors that
run after thread finalization because FuncEntry/Exit and
TraceAddEvent all need the shadow stack.

Reviewed By: vitalybuka

Differential Revision: https://reviews.llvm.org/D113786