ACPI: PM: s2idle: Move x86-specific code to the x86 directory

Some code in drivers/acpi/sleep.c (which is regarded as a generic
file) related to suspend-to-idle support has grown direct dependencies
on x86, but in fact it has been specific to x86 (which is the only
user of it) anyway for a long time.

For this reason, move that code to a separate file under acpi/x86/
and make it build and run as before under the right conditions.

While at it, rename a vendor checking function in that code and
consistently use acpi_handle_debug() for printing debug-related
information in it.

No expected functional impact.

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