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-sparc/fop_helper.c b/target-sparc/fop_helper.c
index f4b62a5..ee4592e 100644
--- a/target-sparc/fop_helper.c
+++ b/target-sparc/fop_helper.c
@@ -18,7 +18,7 @@
  */
 
 #include "cpu.h"
-#include "helper.h"
+#include "exec/helper-proto.h"
 
 #define QT0 (env->qt0)
 #define QT1 (env->qt1)