Add NO_LEGACY_PCIB kernel option to i386, amd64

On systems without a PCI bus, legacy_pcib_identify by default creates
one anyway:

legacy_pcib_identify: no bridge found, adding pcib0 anyway

This commit adds a kernel option NO_LEGACY_PCIB which disables this,
allowing systems to be fully PCI-free.

Reviewed by: imp
Sponsored by: https://www.patreon.com/cperciva
Differential Revision: https://reviews.freebsd.org/D35798