intrng: Add support for multiple interrupt roots

Different types of interrupts may require using different exception
vectors so this commit adds support multiple interrupt roots to handle
these cases. Archs may opt-in to multiple interrupt roots by defining
INTR_ROOT_NUM as the number of roots in their intr.h. Based off
https://reviews.freebsd.org/D40161.

Signed-off-by: Ayrton Munoz <a.munoz3327@gmail.com>
Co-authored-by: Kyle Evans <kevans@FreeBSD.org>
Co-authored-by: Andrew Turner <andrew@FreeBSD.org>
Reviewed-by: imp,mmel,mhorne
Pull-Request: https://github.com/freebsd/freebsd-src/pull/1363