x86/boot: Move startup code out of __head section

Move startup code out of the __head section, now that this no longer has
a special significance. Move everything into .text or .init.text as
appropriate, so that startup code is not kept around unnecessarily.

  [ bp: Fold in hunk to fix 32-bit CPU hotplug:
    Reported-by: kernel test robot <oliver.sang@intel.com>
    Closes: https://lore.kernel.org/oe-lkp/202509022207.56fd97f4-lkp@intel.com

 ]

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/20250828102202.1849035-45-ardb+git@google.com