x86: fix cpu hotplug on 32bit

commit 3e9704739daf46a8ba6593d749c67b5f7cd633d2 ("x86: boot secondary
cpus through initial_code") causes the kernel to crash when a CPU is
brought online after the read only sections have been write
protected. The write to initial_code in do_boot_cpu() fails.

Move inital_code to .cpuinit.data section.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: H. Peter Anvin <hpa@zytor.com>