Fix multilib first with multiple 64-bit architectures

Binaries that use multilib "first" expect zero or one architectures
because they will install to the same bin directory for any
architecture.  Replace preferTargets with firstTarget that always
returns zero or one Target.

Test: OUT_DIR=out_ndk build/soong/scripts/build-ndk-prebuilts.sh
Test: m checkbuild
Change-Id: I6dcfa9ecda30177e1a262153d93a5a205d5c7076