accel/tcg: Honor TLB_DISCARD_WRITE in atomic_mmu_lookup

accel/tcg: Honor TLB_DISCARD_WRITE in atomic_mmu_lookup

Using an atomic write or read-write insn on ROM is basically
a happens-never case.  Handle it via stop-the-world, which
will generate non-atomic serial code, where we can correctly
ignore the write while producing the correct read result.

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>