Fix build of some modules for some kernel configs.

Namely, vmm.ko cannot be compiled without 'option SMP', the code uses
IPIs and LAPIC.
Recently systrace was forced over any configs, check for KDTRACE_HOOK
before compiling the dtrace/ modules.

Reviewed by: markj
Discussed with: mjg
Tested by: se (previous version)
Sponsored by: The FreeBSD Foundation (kib)
Differential revision: https://reviews.freebsd.org/D23699