x86, mcheck: Use correct cpumask for shared bank4

This fixes threshold_bank4 support on multi-node processors.

The correct mask to use is llc_shared_map, representing an internal
node on Magny-Cours.

We need to create 2 sets of symlinks for sibling shared banks -- one
set for each internal node, symlinks of each set should target the
first core on same internal node.

Currently only one set is created where all symlinks are targeting
the first core of the entire socket.

Signed-off-by: Andreas Herrmann <andreas.herrmann3@amd.com>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>