Which codegen backend the building compiler uses is independent of which codegen backend(s) the built compiler uses.
Similarly, you can build Clang using itself or using GCC. The resulting compiler should behave the same and produce the same machine code, even if its own machine code is somewhat different.
The produced binaries could still have artifacts from the original compiler in them, e.g. if "compiler built-in" libraries or standard libraries were compiled with the original compiler.
Both GCC and rustc use a multi-stage build process where the new compiler builds itself again, so you reach an idempotent state where no artifacts from the original compiler are left.
Similarly, you can build Clang using itself or using GCC. The resulting compiler should behave the same and produce the same machine code, even if its own machine code is somewhat different.
The produced binaries could still have artifacts from the original compiler in them, e.g. if "compiler built-in" libraries or standard libraries were compiled with the original compiler.
Both GCC and rustc use a multi-stage build process where the new compiler builds itself again, so you reach an idempotent state where no artifacts from the original compiler are left.