Android: Default to newest toolchain

The recent NDKs have included the 4.9 version of the gcc toolchain,
so we default to that instead of 4.8. This is also required for building
for 64-bit targets, since the other toolchains do not support that.

Change-Id: Id6908eb41c8e7cf003a9b134607ab2e55cb489e9
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@theqtcompany.com>
Reviewed-by: BogDan Vatra <bogdan@kde.org>