libdtrace: add riscv support

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