Mirror the interface of the user-only function of the same name.
Use probe_access_flags for the common case of ram, and
cpu_memory_rw_debug for the uncommon case of mmio.

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
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v3: Use probe_access_flags (pmm)