CtsShim: build tweaks for 64-bit only builds.

It would previously fail to find a matching abi for devices with
TARGET_ARCH=arm64 and no TARGET_2ND_ARCH.

The same apk was used in the 32-bit and 64-bit case anyway.
This package has no code, so it's unclear why it has different
apks for different archs.

Test: lunch sailfish-eng; m -j
Bug: 72810053
Change-Id: I800644ba848118556cd73d28bcf47e279d8eba76