Add support for reading advanced diagnostic counters.

By default reading the diagnostic counters is disabled. The firmware
decides which counters are supported and only those supported show up
in the dev.mce.X.diagnostics sysctl tree.

To enable reading of diagnostic counters set one or more of the
following sysctls to one:

dev.mce.X.conf.diag_general_enable=1
dev.mce.X.conf.diag_pci_enable=1

MFC after: 1 week
Sponsored by: Mellanox Technologies