stackleak: Rename STACKLEAK to KSTACK_ERASE

In preparation for adding Clang sanitizer coverage stack depth tracking
that can support stack depth callbacks:

- Add the new top-level CONFIG_KSTACK_ERASE option which will be
  implemented either with the stackleak GCC plugin, or with the Clang
  stack depth callback support.
- Rename CONFIG_GCC_PLUGIN_STACKLEAK as needed to CONFIG_KSTACK_ERASE,
  but keep it for anything specific to the GCC plugin itself.
- Rename all exposed "STACKLEAK" names and files to "KSTACK_ERASE" (named
  for what it does rather than what it protects against), but leave as
  many of the internals alone as possible to avoid even more churn.

While here, also split "prev_lowest_stack" into CONFIG_KSTACK_ERASE_METRICS,
since that's the only place it is referenced from.

Suggested-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20250717232519.2984886-1-kees@kernel.org
Signed-off-by: Kees Cook <kees@kernel.org>

arch/Kconfig

@@ -630,11 +630,11 @@ config SECCOMP_CACHE_DEBUG
 
 	  If unsure, say N.
 
-config HAVE_ARCH_STACKLEAK
+config HAVE_ARCH_KSTACK_ERASE
 	bool
 	help
 	  An architecture should select this if it has the code which
-	  fills the used part of the kernel stack with the STACKLEAK_POISON
+	  fills the used part of the kernel stack with the KSTACK_ERASE_POISON
 	  value before returning from system calls.
 
 config HAVE_STACKPROTECTOR