x86: Clean up current_stack_pointer

There's no good reason for it to be a macro, and x86_64 will want to
use it, so it should be in a header.

Acked-by: Borislav Petkov <bp@suse.de>
Signed-off-by: Andy Lutomirski <luto@amacapital.net>

diff --cc arch/x86/include/asm/thread_info.h
Simple merge

diff --cc arch/x86/kernel/irq_32.c
Simple merge