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-i386/svm_helper.c b/target-i386/svm_helper.c
index 846eaa5..852e2ba 100644
--- a/target-i386/svm_helper.c
+++ b/target-i386/svm_helper.c
@@ -19,7 +19,7 @@
 
 #include "cpu.h"
 #include "exec/cpu-all.h"
-#include "helper.h"
+#include "exec/helper-proto.h"
 
 #if !defined(CONFIG_USER_ONLY)
 #include "exec/softmmu_exec.h"