Largely untested, as we can't really do anything with user probes
without an implementation of fasttrap. However, this is enough to
generate an embedded dtrace program with `dtrace -G` and link the
generated ELF file.
Reviewed by: markj
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D38301
f711d5c3d0