powerpc: Placeholder and infrastructure/build support to add Power11 related changes.

The following three changes have been added to provide initial Power11 support.
    1. Add the directories to hold Power11 files.
    2. Add support to select Power11 libraries based on AT_PLATFORM.
    3. Let submachine=power11 be set automatically.

Reviewed-by: Florian Weimer <fweimer@redhat.com>
Reviewed-by: Peter Bergner <bergner@linux.ibm.com>