Since we no longer use shared source we can modify the source directory
as we build. This drops another patch we need to build and simplifies
the addition of Rust 1.12.1 in #97.
I'm honestly not sure what this patch does since everything appears to
build the same with and without it. I did a `tree` of the sysroots and
images and I don't see any file differences. Something like this used to
be necessary in Gentoo but I dropped it a few releases back.