Android: detect Linux 64 bit host architecture

This is the easy fix: looking at what is supported by the NDK. If people
have weird setups, then they have to specify -android-ndk-host. We do
actually detect the host architecture later, but using that would be
a much bigger (and riskier) change.

Task-number: QTBUG-31275

Change-Id: I18db878031baa2e1ee2fa4beff364d58d8bd3c7a
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@digia.com>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>