errors: improve the advice to send hwloc-gather-topology files in the OS error message
linux: work around buggy core_siblings on Xeon E5 v3 (Haswell)

Some new Intel processors (Xeon E5 v3 Haswell with 10+ cores) have
two NUMA nodes per socket but the kernel core_siblings topology
file reports two sockets with one node each.
Kernel devs are working at fixing this but some distros will remain
buggy for ever. So we have to work-around it for now since these
processors are already available.

On x86, we now look merge sockets than have the same
physical_package_id but different core_siblings. To avoid modifying
already-inserted socket objects, we queue them on a list, wait until
the end of the sysfs cpu discovery loop, merge things, and insert
the remaining ones.

We can't enable this code on all archs because some ppc and ia64 test
topologies in tests/linux have duplicate socket physical_package_id.
We issue a big warning (that can be hidden with HWLOC_HIDE_ERRORS)
so that people report false-positive. Hopefully no x86 platform
Hopefully, no x86 platform will show duplicate physical_package_id.

Kernel bug is explained here:
 https://bugzilla.redhat.com/show_bug.cgi?id=1141421
 https://lkml.org/lkml/2014/9/15/1240
tests/linux: add a case with buggy core_siblings on Xeon E5 v3

each socket has 14 cores, 2 rings, 2 NUMA nodes