dtrace: expose dtrace_instr_size() to userland and implement it for riscv

dtrace_instr_size() is needed by the forthcoming RISC-V port of kinst,
as well as by libdtrace in D38825 for both amd64 and RISC-V.

Reviewed by: markj, mhorne
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D39489