x86/entry: Disable interrupts for native_load_gs_index() in C code

There is absolutely no point in doing this in ASM code. Move it to C.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Alexandre Chartre <alexandre.chartre@oracle.com>
Acked-by: Peter Zijlstra <peterz@infradead.org>
Acked-by: Andy Lutomirski <luto@kernel.org>
Link: https://lkml.kernel.org/r/20200505134903.531534675@linutronix.de