x86/smpboot: Simplify boot CPU setup

x86/smpboot: Simplify boot CPU setup

Now that the per-cpu GSBASE, stack, and GDT descriptor can be derived
dynamically by CPU number, the boot CPU can use a fixed CPU number and
take the same path as secondary CPUs.

Signed-off-by: Brian Gerst <brgerst@gmail.com>
Reviewed-by: David Woodhouse <dwmw@amazon.co.uk>
Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Tested-by: Usama Arif <usama.arif@bytedance.com>
Signed-off-by: Usama Arif <usama.arif@bytedance.com>
Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>