RISC-V: Various XIP fixes

This fixes a handful of issues with the XIP support, which has bit
rotted some lately.

* palmer/riscv-xip:
  RISC-V: Fix the XIP build
  RISC-V: Split out the XIP fixups into their own file
  RISC-V: ignore xipImage
  RISC-V: Avoid empty create_*_mapping definitions