mm: init: report memory auto-initialization features at boot time

Print the currently enabled stack and heap initialization modes.

Stack initialization is enabled by a config flag, while heap
initialization is configured at boot time with defaults being set in the
config.  It's more convenient for the user to have all information about
these hardening measures in one place at boot, so the user can reason
about the expected behavior of the running system.

The possible options for stack are:
 - "all" for CONFIG_INIT_STACK_ALL;
 - "byref_all" for CONFIG_GCC_PLUGIN_STRUCTLEAK_BYREF_ALL;
 - "byref" for CONFIG_GCC_PLUGIN_STRUCTLEAK_BYREF;
 - "__user" for CONFIG_GCC_PLUGIN_STRUCTLEAK_USER;
 - "off" otherwise.

Depending on the values of init_on_alloc and init_on_free boottime options
we also report "heap alloc" and "heap free" as "on"/"off".

In the init_on_free mode initializing pages at boot time may take a while,
so print a notice about that as well.  This depends on how much memory is
installed, the memory...