FROMLIST: scs: add support for stack usage debugging

Implements CONFIG_DEBUG_STACK_USAGE for shadow stacks. When enabled,
also prints out the highest shadow stack usage per process.

Bug: 145210207
Change-Id: I2b2fea68760ca8d94d6f887cfe5828883d233b88
(am from https://lore.kernel.org/patchwork/patch/1149056/

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