The earlier NDK platforms do not support 64 bit architectures, so
configure would fail with a confusing message about problems in
the environment.

[ChangeLog][Android] Default to android-21 for arm64 builds instead
of failing.

Task-number: QTBUG-70280
Change-Id: Ib9846d6deee3d453fd4a17a3ae92306482d380ba
Reviewed-by: Simon Hausmann <simon.hausmann@qt.io>