x86/percpu: Do not use this_cpu_read_stable_8() for 32-bit targets

this_cpu_read_stable() macro uses __pcpu_size_call_return()
that unconditionally calls this_cpu_read_stable_8() also for
32-bit targets. This usage is ivalid as it will result in the
generation of 64-bit MOVQ instruction on 32-bit targets via
percpu_stable_op() macro.

Since there is no generic support for this_cpu_read_stable_8() for
32-bit targets, the patch defines this_cpu_read_stable_8() to
BUILD_BUG() when CONFIG_X86_64 is not defined. This way, we are
sure that this_cpu_read_stable_8() won't actually be used for
32-bit targets, but it is still defined to prevent build failure.

Signed-off-by: Uros Bizjak <[email protected]>
Signed-off-by: Ingo Molnar <[email protected]>
Cc: Andy Lutomirski <[email protected]>
Cc: Josh Poimboeuf <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Ard Biesheuvel <[email protected]>
Link: https://lore.kernel.org/r/[email protected]

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