x86/entry/64: Use native swapgs in asm_load_gs_index()

When PARAVIRT_XXL is in use, then load_gs_index() uses xen_load_gs_index()
and asm_load_gs_index() is unused.

It's therefore pointless to use the paravirtualized SWAPGS implementation
in asm_load_gs_index(). Switch it to a plain swapgs.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Acked-by: Juergen Gross <jgross@suse.com>
Acked-by: Peter Zijlstra <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20200512213809.583980272@linutronix.de