include/exec: Split out accel/tcg/cpu-mmu-index.h

include/exec: Split out accel/tcg/cpu-mmu-index.h

The implementation of cpu_mmu_index was split between cpu-common.h
and cpu-all.h, depending on CONFIG_USER_ONLY.  We already have the
plumbing common to user and system mode.  Using MMU_USER_IDX
requires the cpu.h for a specific target, and so is restricted to
when we're compiling per-target.

Include the new header only where needed.

Reviewed-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>