According to Eureka team, using protobuf_lib_type=="target" without a direct dependency on a protoc target causes flakiness in the build. This introduces a new variable to represent the protoc host target, with a default value for platforms like iOS where a prebuilt copy of protoc is used. Change-Id: I39c08bd76c6bc4a5291b0e19b2d22a9f3beb04d0