x86/percpu: Introduce the __raw_cpu_read_const() macro

Introduce the __raw_cpu_read_const() macro to further reduce ifdeffery
and differences between configs w/ and w/o USE_X86_SEG_SUPPORT.

No functional change intended.

Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: https://lore.kernel.org/r/20240430091833.196482-4-ubizjak@gmail.com