x86: cleanup handling of 16-byte GDT descriptors

Look them up using a gdt_entry_t pointer, so that the address of
the descriptor is correct even for "odd" selectors (e.g. 0x98).
Rename the struct from segment_desc64 to system_desc64,
highlighting that it is only used in the case of S=0 (system
descriptor).  Rename the "limit" bitfield to "limit2", matching
the convention used for the various parts of the base field.

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