[asan] Reduce flakiness in stack-overflow detection

IsStackOverflow only treats accesses within 512 bytes of SP as stack-overflow. This should really be the size of a page instead.

The scariness_score_test.cc triggers stack overflow with frames that are even larger than a page, which can also trigger a fault that will not be recognized as stack-overflow. Let's just use smaller frames.

llvm-svn: 329980