linux-user: Move includes of target-specific headers to end of qemu.h
The target-specific headers (target_cpu.h and target_signal.h)
might need to use the target-independent structure and function
definitions of qemu.h; so include them only at the bottom of
qemu.h, not the top.
Signed-off-by: Peter Maydell <peter.maydell@linaro.org> Reviewed-by: Richard Henderson <rth@twiddle.net> Signed-off-by: Riku Voipio <riku.voipio@linaro.org>