Allow kernel to be built with stable toolchain.

The only nightly feature still used in the kernel is `#[naked]` on
functions. This can be replaced with careful use of `global_asm!` to
create functions in pure assembly language.

With this change, the kernel builds and works on stable 1.82.0 using my
out-of-tree build system.

build/kernel-link.x

@@ -91,12 +91,14 @@ SECTIONS
     *(.PreResetTrampoline);
     *(.Reset);
 
-    *(.text .text.*);
-
     /* The HardFaultTrampoline uses the `b` instruction to enter `HardFault`,
        so must be placed close to it. */
     *(.HardFaultTrampoline);
     *(.HardFault.*);
+    *(.text.HardFault.*);
+
+    *(.text .text.*);
+
     . = ALIGN(4);
     __etext = .;
   } > FLASH