[asan] initialize fake_stack lazily and increase its maximal size. This makes -fsanitize=address,use-after-return more robust: all SPEC tests pass now. In the default mode thread stacks become a bit smaller.

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@184934 91177308-0d34-0410-b5e6-96231b3b80d8