Allow to omit libc atexit() call

* misc.c (GC_exit_check): Do not define if DONT_USE_ATEXIT (since
unused in this case).
* misc.c (GC_init): Do not call atexit if DONT_USE_ATEXIT (useful if
atexit is missing in libc, e.g., to workaround linkage problems in
Android NDK).
Allow not to rely on __data_start value (Linux)

* os_dep.c (GC_init_linux_data_start): Skip use of [__]data_start if
IGNORE_PROG_DATA_START (useful if give wrong value).
* os_dep.c (GC_init_linux_data_start): Add assertion checks for
[__]data_start to be not greater than _end.