FROMLIST: scs: add accounting

This change adds accounting for the memory allocated for shadow stacks.

Bug: 145210207
Change-Id: I51157fe0b23b4cb28bb33c86a5dfe3ac911296a4
(am from https://lore.kernel.org/patchwork/patch/1149055/

)
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Sami Tolvanen <samitolvanen@google.com>