Use the prebuilt llvm-rs-* if we are doing apps-only build.

This can fix the build error "MODULE.HOST.EXECUTABLES.llvm-rs-cc already
defined by frameworks/compile/slang.  Stop." when you run "tapas" in
full source tree.

Change-Id: I59052d1d4c9c559a739c51d21b623758c41eab17
1 file changed