x86/idt: Make IDT init functions static inlines

Move these two functions from kernel/idt.c to include/asm/desc.h:

	* init_idt_data()
	* idt_init_desc()

These functions are needed to setup IDT entries very early and need to
be called from head64.c. To be usable this early, these functions need
to be compiled without instrumentation and the stack-protector feature.

These features need to be kept enabled for kernel/idt.c, so head64.c
must use its own versions.

 [ bp: Take Kees' suggested patch title and add his Rev-by. ]

Signed-off-by: Joerg Roedel <jroedel@suse.de>
Signed-off-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Kees Cook <keescook@chromium.org>
Link: https://lkml.kernel.org/r/20200907131613.12703-35-joro@8bytes.org