x86: Move 64-bit GDT and TSS to desc.c

Move the GDT and TSS data structures from 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.

Signed-off-by: Zixuan Wang <zixuanwang@google.com>
Message-Id: <20211004204931.1537823-2-zxwang42@gmail.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>