ACPI: processor: Provide empty stub of acpi_proc_quirk_mwait_check()

Commit 0a0e2ea6 ("ACPI: processor: Move MWAIT quirk out of
acpi_processor.c") added acpi_proc_quirk_mwait_check() that is
only defined for x86 and is unlikely to be defined for any other
architectures, so put it under #ifdef CONFIG_X86 and provide
an empty stub implementation of it for the other cases.

This is kind of orthogonal to [1], because if any architectures other
than x86 decide to use the processor _OSC, they will see the reported
build error.

Link: https://lore.kernel.org/lkml/c7a05a44-c0be-46c2-a21d-b242524d482b@roeck-us.net
Link: https://git.kernel.org/pub/scm/linux/kernel/git/ardb/linux.git/commit/?h=remove-ia64&id=a0334bf78b95532cec54f56b53e8ae1bfe7e1ca1

 # [1]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>