Don't depend on @ or machine for assym.s or genassym.o. That's a
vestige of a time when we needed to do that, but it is all handled by
beforedepend now. When we depend on the symlink, bmake will cause the
file to be rebuilt always.

With this change, dtrace.ko doesn't rebuild every time through a
KERNFAST run.

Sponsored by: Netfix
move linux*_locore.s and linux*_support.s to SRCS, remove the OBJS
entry and remove now-redunant dependencies. Add assym.s to
linux*_locore.s build, as it depends on it.

With this change, linux*.ko no longer builds every time through a
KERNFAST run.

Sponsored by: Netflix