x86/percpu: Use %RIP-relative address in untagged_addr()

%RIP-relative addresses are nowadays correctly handled in alternative
instructions, so remove misleading comment and improve assembly to
use %RIP-relative address.

Also, explicitly using %gs: prefix will segfault for non-SMP builds.
Use macros from percpu.h which will DTRT with segment prefix register
as far as SMP/non-SMP builds are concerned.

Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Acked-by: Peter Zijlstra (Intel) <peterz@infradaed.org>
Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: https://lore.kernel.org/all/20231213150357.5942-1-ubizjak%40gmail.com