Build common kernel dependencies before modules.

This ensures files like genassym.o and awk/mfiles are generated before
descending into the modules build. It may also allow some module builds
to not recreate files that are already present in the KERNBUILDDIR.

This fixes a rare build race where genassym.o is missing and assym.inc
is empty.

More work is planned around this to reduce some redundant dependency
generation in modules.

PR: 233339
MFC after: 2 weeks
Reported by: markj