x86: linker: move gdt reserve to outer ifdef

The original CONFIG_GDT_RESERVED_NUM_ENTRIES resides in the false
branch of ifdef LINKER_ZEPHYR_FINAL and is actually elminated
finally. It should reside in the outer ifdef clause, namely the
under ifdef CONFIG_GDT_DYNAMIC.

Signed-off-by: Leifu Zhao <leifu.zhao@intel.com>