tcg: Move qemu_tcg_mttcg_enabled() to 'system/tcg.h'

tcg: Move qemu_tcg_mttcg_enabled() to 'system/tcg.h'

qemu_tcg_mttcg_enabled() is specific to 1/ TCG and
2/ system emulation. Move the prototype declaration
to "system/tcg.h", reducing 'mttcg_enabled' variable
scope.

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-ID: <20250403220420.78937-17-philmd@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>