Add .PHONY for dist-for-goals targets

Mac builds don't include build/make/core/tasks/general-tests.mk
which causes the general-tests target not to be marked phony.  Have
Soong mark all dist-for-goals targets as phony so it doesn't rely
on other makefiles being present.

Bug: 205928834
Test: forrest
Change-Id: I9394c9c794fc83cde9649aa12a3039526d8206f3