unify field names and definitions for GDT descriptors

Use the same names and definitions (apart from the high base field)
for GDT descriptors in both 32-bit and 64-bit code.  The next patch
will also reuse gdt_entry_t in the 16-byte struct definition, for now
some duplication remains.

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