x86_64: Squash initial_code modpost warnings

Get rid of warnings like

WARNING: vmlinux.o(.bootstrap.text+0x1a8): Section mismatch: reference to .init.text:x86_64_start_kernel (between 'initial_code' and 'init_rsp')

- Move initialization code into .text.head like i386 because modpost knows about this already
- Mark initial_code .initdata

Signed-off-by: Andi Kleen <ak@suse.de>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>