sys/intr.h: Make it safe to include from assembler

Sometimes we need defines from this file in assembler code. Today we do
the heavyweight approach of using genassym for that. However, they are
just #defines, so in the future we want to include sys/intr.h to pick up
the needed constants in exception.S.

PR: 283041
Sponsored by: Netflix
Reviewed by: mmel, andrew
Differential Revision: https://reviews.freebsd.org/D47846