ACPICA: Linux-specific header: Add support for PPC64 compilation.

Adds PPC64 as a 64-bit architecture. Colin Ian King.

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Bob Moore <robert.moore@intel.com>
Signed-off-by: Lv Zheng <lv.zheng@intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>