rustbuild: Add support for --enable-local-rust
rustbuild: Add support for --enable-local-rust

This commit adds support for the `--enable-local-rust` `./configure` switch
which uses the locally installed `rustc` to bootstrap the compiler.

graydon/rust,graydon/rust,GBGamer/rust,aidancully/rust,graydon/rust,GBGamer/rust,GBGamer/rust,aidancully/rust,graydon/rust,GBGamer/rust,GBGamer/rust,graydon/rust,aidancully/rust,GBGamer/rust,graydon/rust,GBGamer/rust,aidancully/rust,GBGamer/rust,aidancully/rust,aidancully/rust