Support for aarch64-linux

[koxu1996]

By default rustc0 is released for the following platforms:

• aarch64-apple-darwin
• x86_64-apple-darwin
• x86_64-unknown-linux-gnu

In risc0pkgs, x86_64-unknown-linux-gnu is used for all Linux machines:

risc0pkgs/pkgs/rustc0/default.nix

Lines 47 to 54 in 7acff27

	if stdenv.hostPlatform.isLinux
	then
	builtins.fetchurl
	{
	url = "https://github.com/risc0/rust/releases/download/test-release-2/rust-toolchain-x86_64-unknown-linux-gnu.tar.gz";
	sha256 = "sha256-ilCDZk+YY8lUFqdITR1w1OxBsjNVfUlYUTQDzk2/D9s=";
	}
	else if stdenv.hostPlatform.system == "x86_64-darwin"

I would like to make it compatible with aarch64-linux.

[koxu1996]

I was able to compile toolchain for aarch64-linux, by running the following commands on my Asahi M2 machine:

$ cargo install cargo-binstall
$ cargo binstall cargo-risczero
$ sudo pacman -S cmake
$ cargo risczero build-toolchain
$ cd ~/.risc0/rust/build/aarch64-unknown-linux-gnu/stage2/
$ GZIP=-9 tar -czvf /tmp/rust-toolchain-aarch64-unknown-linux-gnu.tar.gz .

Here is rust-toolchain-aarch64-unknown-linux-gnu.tar.gz (393 MiB, sha256: d9c6a874994adc3dbd5af275d0a4a4672792c099a8bc6358406da0073dc95297).

[marijanp]

Added in #13