x86/boot/64: Put __startup_64() into .head.text

Put __startup_64() and fixup_pointer() into .head.text section to make
sure it's always near startup_64() and always callable.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: kernel test robot <fengguang.wu@intel.com>
Cc: wfg@linux.intel.com
Link: http://lkml.kernel.org/r/20170616113024.ajmif63cmcszry5a@black.fi.intel.com


Signed-off-by: Ingo Molnar <mingo@kernel.org>