x86/head/64: Install a CPU bringup IDT

Add a separate bringup IDT for the CPU bringup code that will be used
until the kernel switches to the idt_table. There are two reasons for a
separate IDT:

	1) When the idt_table is set up and the secondary CPUs are
	   booted, it contains entries (e.g. IST entries) which
	   require certain CPU state to be set up. This includes a
	   working TSS (for IST), MSR_GS_BASE (for stack protector) or
	   CR4.FSGSBASE (for paranoid_entry) path. By using a
	   dedicated IDT for early boot this state need not to be set
	   up early.

	2) The idt_table is static to idt.c, so any function
	   using/modifying must be in idt.c too. That means that all
	   compiler driven instrumentation like tracing or KASAN is
	   also active in this code. But during early CPU bringup the
	   environment is not set up for this instrumentation to work
	   correctly.

To avoid all of these hassles and make early exception handling robust,
use a dedicated bringup IDT.

The IDT is loaded two times, first on the boot CPU while the kernel is
still running on direct mapped addresses, and again later after the
switch to kernel addresses has happened. The second IDT load happens on
the boot and secondary CPUs.

Signed-off-by: Joerg Roedel <jroedel@suse.de>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/20200907131613.12703-34-joro@8bytes.org