tcg: check CF_PARALLEL instead of parallel_cpus

Thereby decoupling the resulting translated code from the current state
of the system.

The tb->cflags field is not passed to tcg generation functions. So
we add a field to TCGContext, storing there a copy of tb->cflags.

Most architectures have <= 32 registers, which results in a 4-byte hole
in TCGContext. Use this hole for the new field.

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