x86/percpu: Construct __percpu_seg_override from __percpu_seg

Construct __percpu_seg_override macro from __percpu_seg by
concatenating the later with __seg_ prefix to reduce ifdeffery.

No functional change intended.

Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: https://lore.kernel.org/r/20250225200235.48007-1-ubizjak@gmail.com