[asan] Unpoison signal alternate stack.

Summary:
Before unwinding the stack, `__asan_handle_no_return` is supposed to
unpoison the entire stack - that is, remove the entries in the shadow
memory corresponding to stack (e.g. redzone markers around variables).
This does not work correctly if `__asan_handle_no_return` is called from
the alternate stack used in signal handlers, because the stack top is
read from a cache, which yields the default stack top instead of the
signal alternate stack top.

It is also possible to jump between the default stack and the signal
alternate stack. Therefore, __asan_handle_no_return needs to unpoison
both.

Reviewers: vitalybuka, kubamracek, kcc, eugenis

Reviewed By: vitalybuka

Subscribers: phosek, #sanitizers

Tags: #sanitizers

Differential Revision: https://reviews.llvm.org/D76986