build: Add a top-level forwarding target for src/* objects
build: Add a top-level forwarding target for src/* objects

Fixes #3955. It's hackish, but seems to always function as expected. Examples:

make src/bitcoind
make src/qt/bitcoin-qt
make src/libbitcoin.a

langerhans/dogecoin,langerhans/dogecoin,langerhans/dogecoin,langerhans/dogecoin,langerhans/dogecoin,langerhans/dogecoin