Make implicit 32-bit apps on 64-bit target an error

This has been a warning for a long time. Make it an error instead.

Once this has been verified to not cause any problems, it can
likely be changed to instead default to setting
TARGET_SUPPORTS_64_BIT_APPS := true.

Test: build_test
Change-Id: I0d03fed2085009c64880a594d5a3aadf72187e60