Add bazel-skylib selects support

In order to handle all of Soong's target.<name> types, use of
selects.config_setting_group simplifies readability as multiple (e.g.
target.bionic, and target.linux) encompass more than one os (android,
linux_bionic and android, linux_bionic, linux_glibc respectively).

Test: build/bazel/ci/bp2build.sh
Change-Id: I65bf098a597a67d585a074efa9852257180b041f