tcg: Rename helper_atomic_*_mmu and provide for user-only

tcg: Rename helper_atomic_*_mmu and provide for user-only

Always provide the atomic interface using TCGMemOpIdx oi
and uintptr_t retaddr.  Rename from helper_* to cpu_* so
as to (mostly) match the exec/cpu_ldst.h functions, and
to emphasize that they are not callable from TCG directly.

Tested-by: Cole Robinson <crobinso@redhat.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>