Kernel/aarch64: Detect if access faults come from SafeMem

This commit lets us differentiate whether access faults are caused by
accessing junk memory addresses given to us by userspace or if we hit a
kernel bug.

The stub implementations of the `safe_*` functions currently don't let
us jump back into them and return a value indicating failure, so we
panic if such a fault happens. Practically, this means that we still
crash, but if the access violation was caused by something else, we take
the usual kernel crash code path and print a register and memory dump,
rather than hitting the `TODO_AARCH64` in `handle_safe_access_fault`.

Kernel/Arch/aarch64/linker.ld

@@ -20,6 +20,14 @@ SECTIONS
     .text ALIGN(4K) : AT (ADDR(.text) - KERNEL_MAPPING_BASE)
     {
         *(.text.first)
+
+        start_of_safemem_text = .;
+        KEEP(*(.text.safemem))
+        end_of_safemem_text = .;
+        start_of_safemem_atomic_text = .;
+        KEEP(*(.text.safemem.atomic))
+        end_of_safemem_atomic_text = .;
+
         *(.text*)
     } :text