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.
Add assertion on number_of_objs to GC_extend_size_map

* misc.c (GC_extend_size_map): Add assertion for number_of_objs (result
of integer division) to be non-zero (this assertion violation is
observed in some Android emulators when gcc -march value does not match
target CPU).
Workaround 'ELF_DATA/EM_ALPHA redefined' warning in Android linker.h

* dyn_load.c: Include asm/elf.h and linux/elf-em.h explicitly, and
undefine ELF_DATA and EM_ALPHA (if PLATFORM_ANDROID and
BIONIC_ELFDATA_REDEF_BUG are defined) to workaround a bug (avoiding
"macro redefined" warnings) in Android 4.1 (and 4.2) Bionic which has
the mismatching macro definitions in headers included from linker.h.
Fix tests makefile to link with new GNU ld linking rules

* tests/tests.am (gctest_LDADD, threadleaktest_LDADD,
threadkey_test_LDADD, subthread_create_LDADD,
initsecondarythread_LDADD): Link to pthreads (or appropriate
threading libraries) as needed; fixes failure to link with new GNU ld
linking rules.
Fix make disclaim_test to link with new GNU ld linking rules

* tests/tests.am (disclaim_test_LDADD): Link to pthreads (or appropriate
threading libraries) as needed.
Fix GC_INIT_CONF_ROOTS in gc.h for Android/x86 and Android/mips

* include/gc.h (GC_INIT_CONF_ROOTS): Do not define to register static
data root in range from __data_start to _end on non-ARM Android
(because __data_start symbol does not exist for x86 and mips); refine
the comment.
Fix AM_CONFIG_HEADER in configure for autoconf-2.69-1

* configure.ac (AM_CONFIG_HEADER): Replace obsolete macro with with
AC_CONFIG_HEADERS one.