As seen here, compiling the Android kernel requires a prebuilt GCC toolchain (or the equivalent from the Android NDK). Cross-compilation makes sense; we are compiling code for a device with a different platform.
However, the guide to compiling the Android source does not anywhere require that one download a toolchain or NDK, and my understanding is that none of the tools used (SDK, etc.) contain a toolchain, either.
But of course compiling the source from scratch also builds the kernel. What's going on -- why do we need a gcc toolchain when building the kernel one way, but not the other?