Remove unused CONFIG_TCG_PASS_AREG0 and dead code

Remove unused CONFIG_TCG_PASS_AREG0 and dead code

Now that CONFIG_TCG_PASS_AREG0 is enabled for all targets,
remove dead code and support for !CONFIG_TCG_PASS_AREG0 case.

Remove dyngen-exec.h and all references to it. Although included by
hw/spapr_hcall.c, it does not seem to use it.

Remove unused HELPER_CFLAGS.

Signed-off-by: Blue Swirl <blauwirbel@gmail.com>
Reviewed-by: Richard Henderson <rth@twiddle.net>