x86: Introduce GDT_ENTRY_INIT()

GDT_ENTRY_INIT is static initializer of desc_struct.

We already have similar macro GDT_ENTRY() but it's static
initializer for u64 and it cannot be used for desc_struct.

Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
LKML-Reference: <20090718151219.GD11294@localhost.localdomain>
Signed-off-by: Ingo Molnar <mingo@elte.hu>