arch_topology: Fix compilation for possible sibling mask

Commit'd11f209b("arch_topology: Add possible sibling cpu mask for
cpu_topology")' introduced core_possible_sibling_mask, which is not
needed for 32-bit architectures and throws compilation error for 32-bit.

Fix it by returning core_sibling_mask for core_possible_sibling_mask.

Change-Id: I559f786f88773fe9d409c9a329b5a1cee7287700
Signed-off-by: Lingutla Chandrasekhar <clingutla@codeaurora.org>