riscv: Drop const annotation for sp

The const annotation should not be used for 'sp', or it will
become read only and lead to bad stack output.

Fixes: dec8227 ("riscv: stacktrace: Move register keyword to beginning of declaration")
Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>

Author: Kefeng Wang

arch/riscv/kernel/stacktrace.c

@@ -14,7 +14,7 @@
 
 #include <asm/stacktrace.h>
 
-register const unsigned long sp_in_global __asm__("sp");
+register unsigned long sp_in_global __asm__("sp");
 
 #ifdef CONFIG_FRAME_POINTER