Build all files matching *_helper.c with HELPER_CFLAGS, not just
op_helper.c. This allows you to put target helper functions which
use the global 'env' variable in multiple source files.

This only affects the ARM target as all the other targets currently only
have op_helper.c.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>