Groundwork for supporting multiple TCG contexts.

Compile-tested for all targets on an x86_64 host.

Suggested-by: Richard Henderson <rth@twiddle.net>
Acked-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: Emilio G. Cota <cota@braap.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>