Fix 'wrong __data_start/_end pair' error on Android

Fix 'wrong __data_start/_end pair' error on Android
(fix of commits b746e637b, 5e73ff184, a25965b5f)

Issue #259 (bdwgc).

* include/gc.h [(HOST_ANDROID || __ANDROID__)
&& IGNORE_DYNAMIC_LOADING] (_etext, __data_start, __end__, _end): Do
not declare weak symbols.
* os_dep.c [SEARCH_FOR_DATA_START && (LINUX || HURD) && HOST_ANDROID]
(etext, __dso_handle): Likewise.
* include/gc.h [(HOST_ANDROID || __ANDROID__)
&& IGNORE_DYNAMIC_LOADING] (GC_find_limit): Declare function as public.
* include/gc.h [(HOST_ANDROID || __ANDROID__)
&& IGNORE_DYNAMIC_LOADING] (GC_INIT_CONF_ROOTS): Update comment;
do not use _etext, __data_start, __end__, _end symbols; use
GC_find_limit(__dso_handle,1) as the end of the added GC data root.
* include/private/gc_priv.h [SEARCH_FOR_DATA_START
|| NETBSD && __ELF__] (GC_find_limit): Change ptr_t type to void*,
GC_bool to int.
* include/private/gcconfig.h [(SPARC || ALPHA) && FREEBSD]
(GC_find_limit): Likewise.
* os_dep.c [NEED_FIND_LIMIT || USE_PROC_FOR_LIBRARIES] (GC_find_limit):
Likewise.
* include/private/gcconfig.h [(SPARC || ALPHA) && FREEBSD]
(DATAEND): Cast the result to ptr_t.
* include/private/gcconfig.h [AARCH64 && LINUX && HOST_ANDROID]
(SEARCH_FOR_DATA_START): Remove outdated comment about __data_start.
* os_dep.c [SEARCH_FOR_DATA_START && (LINUX || HURD)
&& !IGNORE_PROG_DATA_START && HOST_ANDROID && !CPPCHECK]
(GC_init_linux_data_start): Do not compare __dso_handle to _etext and
do not use __dso_handle as data start.
* os_dep.c [SEARCH_FOR_DATA_START] (GC_init_linux_data_start): Cast the
result of GC_find_limit() to ptr_t.
* os_dep.c [NETBSD && __ELF__] (GC_init_netbsd_elf): Likewise.
* os_dep.c [LINUX_STACKBOTTOM && IA64] (GC_get_register_stack_base):
Likewise.
* os_dep.c [!AMIGA && !HAIKU && !OS2 && !MSWIN32 && !MSWINCE
&& !CYGWIN32 && !GC_OPENBSD_THREADS && (!GC_SOLARIS_THREADS
|| _STRICT_STDC)] (GC_get_main_stack_base): Likewise.
* os_dep.c [DATASTART_USES_BSDGETDATASTART] (GC_FreeBSDGetDataStart):
Likewise.