dtrace: Fix RISC-V user stack unwinder

The unwind logic was copied from AArch64 which follows the peculiar
AACPS (where, unlike typical RISC architectures, its frame pointer
follows an x86/stack machine-like convention where the frame pointer
points at the bottom of the frame record, not the top). Delete the
pointless riscv_frame struct and fix this.

Reviewed by: mhorne
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D28054