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: [e-mail]
Signed-off-by: Kees Cook <[e-mail]>
Acked-by: Geert Uytterhoeven <[e-mail]>
Link: https://lore.kernel.org/lkml/CAMuHMdU6msvi0j=[e-mail]

arch/m68k/Kconfig

···
 	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

···
 
 #define	current	get_current()
 
-#endif /* CONFNIG_MMU */
+#endif /* CONFIG_MMU */
+
+register unsigned long current_stack_pointer __asm__("sp");
 
 #endif /* !(_M68K_CURRENT_H) */