[should rebase & merge] 6 commits for align nemu with nutshell - #286
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The four pmpcfg/pmpaddr stubs were writable but never checked. OpenSBI 1.9 sees them and fails isolation. Removing the CSRs is spec-legal (zero PMP) and lets firmware treat PMP as absent.
medeleg[11] (environment call from M-mode) was writable. The privileged spec makes it read-only zero; it cannot be delegated. Bits 18 and 19 (software-check / hardware-error) stay WARL even though NutShell does not implement those exceptions, so firmware writes to the full mask are not dropped.
The spec allows xtval to be 0 or the faulting PC on a breakpoint exception. Write the PC so DiffTest matches NEMU.
Do not implement the unprivileged cycle/time/instret CSRs. Keep mcounteren/scounteren bits 0-2 writable so OpenSBI can trap-and-emulate rdtime. Leave HPM bits read-only zero.
U-mode requires the envcfg CSRs. NutShell implements none of the controlled extensions, so every field is hardwired zero. mseccfg stays unimplemented.
Rebuild from NEMU riscv64-nutshell-ref on master plus the NutShell CSR PRs: no PMP/PMA/Zicntr/Zihpm/mseccfg, keep menvcfg/senvcfg, and CY/TM/IR writable in mcounteren.
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