pci: Add hw.pci.intx_reroute sysctl/tunable

INTRng can leak resources when INTx interrupts are re-routed, which is
typically harmless but can be fatal when devices are (repeatedly) hot
plugged into PCI buses on INTRng systems. Re-routing INTx interrupts
is nonetheless still necessary on some systems, and identifying whether
the re-routing should be enabled or disabled seems to be nontrivial.

Add a hw.pci.intx_reroute sysctl/tunable so systems which don't want
legacy PCI interrupt re-routing can turn it off. This is probably not
the best fix but it's something which can be safely included in FreeBSD
14.3.

Co-Authored-by: jhb
Reviewed by: bz, jhb
MFC after: 3 days
Sponsored by: Amazon
Differential Revision: https://reviews.freebsd.org/D49849