move away TCG-only code, make it compile only on TCG.

Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
[claudio: moved the prototypes from hw/core/cpu.h to exec/cpu-all.h]
Signed-off-by: Claudio Fontana <cfontana@suse.de>
Message-Id: <20210204163931.7358-4-cfontana@suse.de>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>