android: pointer tagging countermeasure.

We strip Android's pointer tag from our heap
pointer while we own it, then put it back at
free time.

* configure (android_target): New variable.
Set this to y in the test where we detect Android.
When setting CONFIG_NAN_BOXING, also set
CONFIG_NAN_BOXING_STRIP_TAG if on Android.

* gc.c (struct heap): New member, tag.
(more): When tag stripping is enabled, clear the
top 16 bits of the pointer coming from malloc,
and keep those bits in heap->tag. This gets rid
of Android's tag.
(sweep): When releasing a heap block with free,
we must put the tag back into the pointer, from
heap->tag.

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