x86: Move IDT to desc.c

Move the IDT data structures from x86/cstart.S and x86/cstart64.S to
lib/x86/desc.c, so that the follow-up UEFI support commits can reuse
these definitions, without re-defining them in UEFI's boot up assembly
code.

Extracted by a patch by Zixuan Wang <zxwang42@gmail.com> and ported
to 32-bit too.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>