[IA64] Only build arch/ia64/kernel/acpi.o when CONFIG_ACPI

The following commit broke the ia64 sim_defconfig build:
	3b2b84c0b81108a9a869a88bf2beeb5a95d81dd1
	ACPI: processor: driver doesn't need to evaluate _PDC

This is because it added:
	+#include <acpi/processor.h>

To arch/ia64/kernel/acpi.c. Unfortunately, the ia64_simdefconfig does
not turn on CONFIG_ACPI, and we get build errors.

The fix described in $subject seems to be the most sensible way to
untangle the mess.

The other issue is that acpi_get_sysname() is required for all configs,
most of which define CONFIG_ACPI, but are not CONFIG_IA64_GENERIC. Turn
it into an inline to cover the "non generic" ia64 configs; to prevent
a duplicate definition build error, we need to wrap the definition in
acpi.o inside an #ifdef.

Finally, move the pm_idle and pm_power_off exports into process.c (which
is always built), similar to other architectures, and allow the sim
defconfig to link.

Signed-off-by: Alex Chiang <achiang@hp.com>
Signed-off-by: Tony Luck <tony.luck@intel.com>