Add blinker03.cbackend.elf target for roundtrip compile via C backend.

This target compiles: C source -> .bc -> optimize .bc ->
compile .bc to C via C backend -> .bc -> optimize -> .s -> .elf.

This target is essentially for studying C codegeneration backend of
LLVM. Note that C backend was removed in LLVM 3.1, so LLVM 3.0 should be
used.