tcg: Invert the inclusion of helper.h

Rather than include helper.h with N values of GEN_HELPER, include a
secondary file that sets up the macros to include helper.h.  This
minimizes the files that must be rebuilt when changing the macros
for file N.

Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Richard Henderson <rth@twiddle.net>
diff --git a/target-ppc/misc_helper.c b/target-ppc/misc_helper.c
index 2eb2fa6..7331b1b 100644
--- a/target-ppc/misc_helper.c
+++ b/target-ppc/misc_helper.c
@@ -17,7 +17,7 @@
  * License along with this library; if not, see <http://www.gnu.org/licenses/>.
  */
 #include "cpu.h"
-#include "helper.h"
+#include "exec/helper-proto.h"
 
 #include "helper_regs.h"