Refactor and fix LR/SC implementation (#217)

- Use physical addresses to avoid homonym ambiguity (closes #215)

- Yield reservation on store-conditional (https://github.com/riscv/riscv-isa-manual/commit/03a5e722fc0fe7b94dd0a49f550ff7b41a63f612)

- Don't yield reservation on exceptions (it's no longer required).