arch: arm64: detect kernel stack overflow condition

Currently, when a kernel stack overflow occures, the exception handler
blindly attempts to write the current context to the stack. If the SP
isn't valid this causes another fault, and so on - locking up the
system.

This commit re-arranges the stack layout, performs SP validation before
usage and switches to an emergency stack when SP isn't valid. This
allows the handler to run and panic gracefully.

Fixes: #98

libkernel/src/memory/address.rs

@@ -152,7 +152,7 @@ impl<K: MemKind, T> Address<K, T> {
     }
 
     #[must_use]
-    pub fn add_bytes(self, n: usize) -> Self {
+    pub const fn add_bytes(self, n: usize) -> Self {
         Self::from_value(self.value() + n)
     }
 

libkernel/src/memory/region.rs

@@ -64,8 +64,8 @@ impl<T: MemKind> MemoryRegion<T> {
     /// Create a memory region from a start and end address.
     ///
     /// The size is calculated as `end - start`. No alignment is enforced.
-    pub fn from_start_end_address(start: Address<T, ()>, end: Address<T, ()>) -> Self {
-        assert!(end >= start);
+    pub const fn from_start_end_address(start: Address<T, ()>, end: Address<T, ()>) -> Self {
+        assert!(end.value() >= start.value());
 
         Self {
             address: start,