tcg: Add CONFIG_ATOMIC64

tcg: Add CONFIG_ATOMIC64

Allow qemu to build on 32-bit hosts without 64-bit atomic ops.

Even if we only allow 32-bit hosts to multi-thread emulate 32-bit
guests, we still need some way to handle the 32-bit guest using a
64-bit atomic operation.  Do so by dropping back to single-step.

Reviewed-by: Emilio G. Cota <[email protected]>
Reviewed-by: Alex Bennée <[email protected]>
Signed-off-by: Richard Henderson <[email protected]>