x86: drop -fno-stack-protector annotations after pt_regs fixes

Now that no functions rely on struct pt_regs being passed by value,
various "no stack protector" annotations can be dropped.

Signed-off-by: Brian Gerst <brgerst@gmail.com>
Acked-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>