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-m68k/translate.c b/target-m68k/translate.c
index cd66289..fa248d9 100644
--- a/target-m68k/translate.c
+++ b/target-m68k/translate.c
@@ -23,9 +23,8 @@
 #include "tcg-op.h"
 #include "qemu/log.h"
 
-#include "helper.h"
-#define GEN_HELPER 1
-#include "helper.h"
+#include "exec/helper-proto.h"
+#include "exec/helper-gen.h"
 
 //#define DEBUG_DISPATCH 1