add -SafeStack bots

These bots use https://clang.llvm.org/docs/SafeStack.html.

This separates the safe (constant size?) parts of the stack like return
address and some local variables apart from the unsafe over- and
underflow prone array uses.  These go on the "unsafe stack".

In theory this makes sense on {Linux,Mac}x{Debug,Release},
but let's just start with Linux/Debug.

Change-Id: I9f5cbeb7081e7d9ac8e1298ca0d86880a6085cfa
Reviewed-on: https://skia-review.googlesource.com/62743
Reviewed-by: Yuqian Li <liyuqian@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>
5 files changed