dtrace: deduplicate some RISC-V functions

match_opcode() is defined in FBT, kinst, and dtrace_subr.c. The function
prologue-checking functions are defined in FBT and kinst.

Reviewed by: markj
Approved by: markj (mentor)
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D40335