util.c: Stop ck_index/fbm_compile from abusing readonliness

util.c: Stop ck_index/fbm_compile from abusing readonliness

There is currently a convenient exception in sv_force_normal that
allows read-only scalars to be modified at compile time.

I want to get rid of it, since most code that relies on it is buggy.

So stop ck_index from relying on that when it calls fbm_compile by
stopping fbm_compile from triggering the error by forcing stringifica-
tion of variables that are already strings.