x86/boot: install trap handlers much earlier on boot

x86/boot: install trap handlers much earlier on boot

Patch the trap handlers into the master idt very early on boot, and setup &
load the GDT, IDT, TR and LDT. Load the IDT before the TR so we stand a chance
of catching an invalid TSS exception rather than triple faulting.

This provides full exception support far earlier on boot than previously.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>