Firstly, oldfp was unitialised, as discovered in f222a6b88614, so start
off pointing at the current frame pointer like other architctures.
Secondly, the actual 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.