riscv: Rename "sp_in_global" to "current_stack_pointer"

To follow the existing per-arch conventions, rename "sp_in_global" to
"current_stack_pointer". This will let it be used in non-arch places
(like HARDENED_USERCOPY).

Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>