Add support of Android logger

Add support of Android logger

* doc/README.macros (GC_ANDROID_LOG): Document new macro.
* misc.c: Include android/log.h if GC_ANDROID_LOG.
* misc.c (GC_ANDROID_LOG_TAG): Define new macro if GC_ANDROID_LOG (and
not ye defined).
* misc.c (GC_printf, GC_log_printf, GC_err_puts): Output message using
__android_log_write with ANDROID_LOG_DEBUG/INFO/ERROR level
(respectively) and GC_ANDROID_LOG_TAG logger name if GC_ANDROID_LOG
(skip writing to GC_stdout/stderr/log (respectively) in this case
unless redirected to a file).
* misc.c (GC_default_on_abort): If GC_ANDROID_LOG then invoke
__android_log_assert after WRITE with the same message except for
omitting redundant "\n" (and ignore GC_LOOP_ON_ABORT checking in this
case because android_log_assert is a no-return function).