build: remove old cgo files

Cgo changed to write these files into _obj, but some
trees may still have the old ones in the source directory.
They need to be removed during make clean so that
a subsequent build will use the ones in _obj.

R=r, r2
CC=golang-dev
https://golang.org/cl/4254056

src/Make.pkg

@@ -47,7 +47,7 @@ coverage:
 	gotest
 	6cov -g $(shell pwd) $O.out | grep -v '_test\.go:'
 
-CLEANFILES+=*.so _obj _test _testmain.go *.exe _cgo*
+CLEANFILES+=*.so _obj _test _testmain.go *.exe _cgo* *.cgo[12].*
 
 test:
 	gotest