Hi Guix!

Good news everyone, I managed to get a aarch64 build of Rust by updating
mrustc to 0.10!  Sadly, i686 is still unsupported, due to the build
using too much memory, it must be the same for other 32-bit architectures.

I tested this on a pinebookpro which only has 4G of RAM, I had to attach
16G of swap for the initial rust@1.39 build to succeed, then the whole
rust chain took about 5 days!  I also tested on my x86_64 desktop.

Here's the patch! After it we should be able to follow-up and enable
Gnome and friends.

That being said, I wasn't entirely sure where this patch should go,
ideally it would be good to have a staging branch rather than
core-updates, WDYT?  Maybe we can do this along with a rust version
update?

PS: I also tried to build rust 1.54 with the new mrustc update, to
shorten the chain, but I've not had any successes with it yet.