x86/head/64: Mark 'startup_gdt[]' and 'startup_gdt_descr' as __initdata

As 'startup_gdt[]' and 'startup_gdt_descr' are only used in booting,
mark them as __initdata to allow them to be freed after boot.

Signed-off-by: Hou Wenlong <houwenlong.hwl@antgroup.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/c85903a7cfad37d14a7e5a4df9fc7119a3669fb3.1689130310.git.houwenlong.hwl@antgroup.com