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
