Workaround invalid '_end' symbol on Android clang 3.5+

Workaround invalid '_end' symbol on Android clang 3.5+

* include/gc.h (GC_INIT_CONF_ROOTS): Handle Android clang (3.5-3.6)
case when "_end" symbol has incorrect address but "__end__" is defined
(by bfd linker only); declare "__end__" symbol as weak.
* include/private/gcconfig.h (DATAEND): Redefine for Android/clang;
define __end__ as weak symbol.
* os_dep.c (_end): Remove declaration (declared in gcconfig.h).
* os_dep.c (GC_init_linux_data_start): Define new local variable
(data_end); set it to DATAEND; use it instead of _end.