i386: Fix start_kernel warning

Fix

WARNING: vmlinux.o(.text+0x183): Section mismatch: reference to .init.text:start_kernel (between 'is386' and 'check_x87')

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