m68k: Implement "current_stack_pointer"

To follow the existing per-arch conventions, add asm "sp" as
"current_stack_pointer". This will let it be used in non-arch places
(like HARDENED_USERCOPY).

Cc: linux-m68k@lists.linux-m68k.org
Signed-off-by: Kees Cook <keescook@chromium.org>
Acked-by: Geert Uytterhoeven <geert@linux-m68k.org>
Link: https://lore.kernel.org/lkml/CAMuHMdU6msvi0j=mS28GFYbm+uMRk7PkYe+zOM4sDmOVxeibLQ@mail.gmail.com

arch/m68k/Kconfig

@@ -4,6 +4,7 @@ config M68K
 	default y
 	select ARCH_32BIT_OFF_T
 	select ARCH_HAS_BINFMT_FLAT
+	select ARCH_HAS_CURRENT_STACK_POINTER
 	select ARCH_HAS_DMA_PREP_COHERENT if HAS_DMA && MMU && !COLDFIRE
 	select ARCH_HAS_SYNC_DMA_FOR_DEVICE if HAS_DMA
 	select ARCH_HAVE_NMI_SAFE_CMPXCHG if RMW_INSNS

arch/m68k/include/asm/current.h

@@ -24,6 +24,8 @@ static inline struct task_struct *get_current(void)
 
 #define	current	get_current()
 
-#endif /* CONFNIG_MMU */
+#endif /* CONFIG_MMU */
+
+register unsigned long current_stack_pointer __asm__("sp");
 
 #endif /* !(_M68K_CURRENT_H) */