Support build for Android 64-bit (arm64, mips64, x86_64)

Support build for Android 64-bit (arm64, mips64, x86_64)

* include/private/gcconfig.h (_end): Replace type from char[] to int[]
to avoid redefinition error (as it is also declared in gc.h) for
Linux/x86_64.
* include/private/gcconfig.h (NO_GETCONTEXT): Define for all Android
targets.
* include/private/gcconfig.h (USE_TKILL_ON_ANDROID): New macro
(defined only for 32-bit Android targets).
* pthread_stop_world.c (tkill, android_thread_kill, GC_suspend_all,
GC_start_world): Use USE_TKILL_ON_ANDROID instead of PLATFORM_ANDROID.