Flit Error Counter ERR_COR Signaling

This change strongly recommends that ERR_COR signaling is supported when the Flit Logging Extended Capability is supported, regardless of whether Advanced Error Reporting (AER) is supported. It defines a method to generate an ERR_COR message when the Flit Error Counter meets a predetermined threshold. Currently, only an interrupt (i.e., MSI or INTx) can be generated when the error threshold is met. The change adds the appropriate capability/control/status bits to enable the ERR_COR generation, with one of two different Subclasses (i.e., SIG_SFW or SIG_OS), when the appropriate conditions are met. A second Flit Logging Extended Capability Version is defined so that implementations can indicate support for this change.