Check DATASTART is less than DATAEND even assertions off

Check DATASTART is less than DATAEND even assertions off

* dyn_load.c (GC_register_dynamic_libraries_dl_iterate_phdr):
Abort with the appropriate message if DATASTART is NULL or greater than
DATAEND (same for DATASTART2 and DATAEND2).
* os_dep.c (GC_init_linux_data_start, GC_register_data_segments):
Likewise.
* os_dep.c (GC_init_linux_data_start, GC_register_data_segments):
Remove assertion about DATASTART and DATAEND.
* os_dep.c (GC_init_linux_data_start): Code refactoring.