* calling setjmp() at the beginning of exec() will reduce performance

* If an RVC instruction is at the end of page boundary, we should not
  fetch 4 bytes. Otherwise, we may trigger an IPF when fetching the
  remaining 2 bytes at the beginning of the next page, which is
  inconsistent with the behavior of the RISC-V priviledged manual.

* In such a case, we should first fetch 2 byte. If it is an RVC
  instruction, then execute it without fetching the remaining 2 bytes.
  If it is not an RVC instruciton, then fetching the remaining 2 bytes
  to obtain a 4-byte instruction.

* With RVC, an instruction may cross page boundary. To fix this, we
  should fetch twice.