platform_ToolchainOptions: check for executable stack

Adds a check for executable stacks, adds some comments, and adds the
ability to add "#" lines to whitelist files.

BUG=None
TEST=passes on x86-alex

Change-Id: I54ac36996f2cd4ac3535e3948123c7681e618565
Signed-off-by: Kees Cook <keescook@chromium.org>
Reviewed-on: https://gerrit.chromium.org/gerrit/21811
2 files changed