How to use custom-build rustc to build WebAssembly?

I tried to use compiled rustc to compile webassembly but failed. I add custom build rustc to toolchain and tried to add wasm32-unknown-unknown target but get error message below:

rustup target add --toolchain stage2 wasm32-unknown-unknown
error: toolchain 'stage2' does not support components
error: caused by: invalid toolchain name: 'stage2'

It seems that custom toolchain don't support components adding. And I can't find any related issue and document, even rustc-guide doesn't mention it. Could someone give me a hint?