init_per_cpu_var() returns a pointer in the percpu address space while
rip_rel_ptr() expects a pointer in the generic address space.

When strict address space checks are enabled, GCC's named address space
checks fail:

  asm.h:124:63: error: passing argument 1 of 'rip_rel_ptr' from
                       pointer to non-enclosed address space

Add a explicit cast to remove address space of the returned pointer.

Fixes: 11e36b0f ("x86/boot/64: Load the final kernel GDT during early boot directly, remove startup_gdt[]")
Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/all/20240819083334.148536-1-ubizjak@gmail.com