Merge branch 'difftest' into main

Set `a0` to zero if tests pass, which is required by TB.

Remove RNMI init since it is not support in DUT.

Change terminate instruction from ecall/scall to `0x6b` for difftest TB.

Cope with presence of Smrnmi extension

Update encoding.h for Smrnmi definitions

Increase v env stack size

env: trap and page fault filter mechanism (#40)

Reverse memcpy direction when evicts a page. (#34)

update riscv_arch.h to support QEMU (#31)

Update encoding.h to add new PTE_ macros

Fix __clear_cache(0, 0) compilation issue (#30)

Merge pull request #27 from bucaps/satp_stval-fixes

v/entry.S: replace sbadaddr with stval

Replace sptbr with satp throughout