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: NAlex Bennée <alex.bennee@linaro.org>
Signed-off-by: NRichard Henderson <rth@twiddle.net>