create a new top-level makefile target "clean.internal"
create a new top-level makefile target "clean.internal"

so I can clean just the non-external stuff when re-running my
clang plugins

Change-Id: I251ceba7e57454a91da5b1082192bb2d211e5406

JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core