Fix GC_init_linux_data_start to set GC_data_start to valid address

Fix GC_init_linux_data_start to set GC_data_start to valid address
even if GC_no_dls
(fix commit db0da19)

* os_dep.c (GC_init_linux_data_start): Test GC_no_dls only to avoid
GC_find_limit call; initialize GC_data_start to DATAEND in case of
GC_no_dls to make data root empty (to prevent incorrect argument
passed to GC_add_roots_inner call in GC_register_data_segments).
* os_dep.c (GC_register_data_segments): Do not invoke
GC_add_roots_inner if upper bound (obtained from sbrk) is not upper
than DATASTART (e.g., if the latter is set to DATAEND on GC_no_dls).