alpha: Implement "current_stack_pointer"

To follow the existing per-arch conventions replace open-coded use
of asm "$30" as "current_stack_pointer". This will let it be used in
non-arch places (like HARDENED_USERCOPY).

Cc: Richard Henderson <[e-mail]>
Cc: Ivan Kokshaysky <[e-mail]>
Cc: Matt Turner <[e-mail]>
Cc: Geert Uytterhoeven <[e-mail]>
Cc: Andrew Morton <[e-mail]>
Cc: Mike Rapoport <[e-mail]>
Cc: Mark Rutland <[e-mail]>
Cc: "Peter Zijlstra (Intel)" <[e-mail]>
Cc: Kefeng Wang <[e-mail]>
Cc: "Alexander A. Klimov" <[e-mail]>
Cc: [e-mail]
Signed-off-by: Kees Cook <[e-mail]>
Signed-off-by: Matt Turner <[e-mail]>

arch/alpha/Kconfig

···
 	bool
 	default y
 	select ARCH_32BIT_USTAT_F_TINODE
+	select ARCH_HAS_CURRENT_STACK_POINTER
 	select ARCH_MIGHT_HAVE_PC_PARPORT
 	select ARCH_MIGHT_HAVE_PC_SERIO
 	select ARCH_NO_PREEMPT

arch/alpha/include/asm/thread_info.h

···
 register struct thread_info *__current_thread_info __asm__("$8");
 #define current_thread_info()  __current_thread_info
 
+register unsigned long *current_stack_pointer __asm__ ("$30");
+
 #endif /* __ASSEMBLY__ */
 
 /* Thread information allocation.  */

arch/alpha/lib/stacktrace.c

···
 {
 	instr * ret_pc;
 	instr * prologue = (instr *)stacktrace;
-	register unsigned char * sp __asm__ ("$30");
+	unsigned char *sp = (unsigned char *)current_stack_pointer;
 
 	printk("\tstack trace:\n");
 	do {