Support host phony modules

I'm fixing make to differentiate between host and target, so this will
be more useful soon.

Test: Android-aosp_arm64.mk is the same before/after
Test: adding `host_supported: true` to shell_and_utilities_system produces expected Android-aosp_arm64.mk
Change-Id: Idf12703be692100336db6d5c0ec05bb96e5e5c27