load.c to use mrb_ro_data_p()
use __init_array_start to determine readonly data section;

b72e94f used _etext and _edata to distinguish C string literals from heap allocated strings,
but using _edata, global char arrays may be considered as string literals.  to avoid the issue,
we have to use __init_array_start, which might be less portable.  you can still use _edata, by
using MRB_NO_INIT_ARRAY_START.