x86/percpu: Introduce raw_cpu_read_long() to reduce ifdeffery

Introduce raw_cpu_read_long() macro to slightly reduce ifdeffery
in <asm/percpu.h>.

No functional changes intended.

Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: https://lore.kernel.org/r/20240404094218.448963-3-ubizjak@gmail.com