Fix trapframe
Fix ASID setting
User space init can now execute its first instructions

- The context (regs) saved upon exception are now
  saved according to struct trapframe layout
- Flush icache and set ITLB entry when exception
  on "unsynched" page
- handle VM_PROT_EXECUTE and VM_PROT_READ in lm32_trap
  VM_PROT_WRITE still needs to be detected somehow