x86/percpu: Define PER_CPU_VAR macro also for !__ASSEMBLY__

Some C source files define 'asm' statements that use PER_CPU_VAR,
so make PER_CPU_VAR macro available also without __ASSEMBLY__.

Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20231105213731.1878100-2-ubizjak@gmail.com