linux_bionic_arm64 is added when Host_bionic_arm64 is true

linux_bionic_arm64 is the new host target using Bionic as libc.  This
change adds a new Soong config Host_bionic_arm64 that enables the
target.

This change also amends firstTarget() function to handle the case when
the available targets for an OS have different arch families. In that
case, first target is selected for each of the arch family. This is
needed in the (rare) care when both linux_bionic_x86_64 and
linux_bionic_arm64 targets are enabled.

Bug: 159685774
Test: add "Host_bionic_arm64": true to out/soong/soong.config and run
./build/soong/soong_ui.bash --make-mode --skip-make
out/soong/host/linux_bionic-arm64/bin/aarch64-linux-bionic/crosvm

Change-Id: I0885e49379f6ee216c6e82adbb1230655171a7a5