[Bug middle-end/69976] Zero the local stack on function exit

[dmalcolm at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69976

--- Comment #9 from David Malcolm <dmalcolm at gcc dot gnu.org> ---
Created attachment 37972
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=37972&action=edit
Crude test case

In case it's helpful, here's a crude attempt at a black-box test case for this:
read a secret into an auto buffer in one function, and then see if it's
readable in another function.

Currently fails with exit code 2 on trunk (with warning ‘security_sensitive’
attribute directive ignored [-Wattributes])

(Naturally we'd want white-box testing also, to verify that the generated code
contains zeroing code).