packaging: make System.map mode 0600

packaging: make System.map mode 0600

To complement the 0400 /proc/kallsyms patch, this makes the installed
System.map file mode 0600 so that security vulnerability exploitation
isn't as trivial. This, like kallsyms, does not stop a serious attacker,
since they can always just fetch the package and read the file.

I'm not aware of any non-root consumer of this file, so there should be
no impact. FWIW, my system boots fine with this change.

Signed-off-by: Kees Cook <kees.cook@canonical.com>
Signed-off-by: Tim Gardner <tim.gardner@canonical.com>