x86: parsing for ACPI "SAPIC" table

Add kernel support for new ACPI "sapic" tables that contain 16-bit APICIDs.
This patch simply adds parsing of an optional SAPIC table if present.
Otherwise, the traditional local APIC table is used.

Note: the SAPIC table is not a new ACPI table - it exists on other architectures
but is not currently recognized by x86_64.

Signed-off-by: Jack Steiner <steiner@sgi.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>