crystal: support aarch64-linux

Upstream does not publish aarch64-linux bootstrap binaries, so grab
them from Alpine

pkgs/development/compilers/crystal/default.nix

@@ -33,6 +33,7 @@ let
     i686-linux = "linux-i686";
     x86_64-darwin = "darwin-x86_64";
     aarch64-darwin = "darwin-universal";
+    aarch64-linux = "linux-aarch64";
   };
 
   arch = archs.${stdenv.system} or (throw "system ${stdenv.system} not supported");
@@ -40,13 +41,19 @@ let
 
   checkInputs = [ git gmp openssl readline libxml2 libyaml ];
 
+  binaryUrl = version: rel:
+    if arch == archs.aarch64-linux then
+      "https://dev.alpinelinux.org/archive/crystal/crystal-${version}-aarch64-alpine-linux-musl.tar.gz"
+    else
+      "https://github.com/crystal-lang/crystal/releases/download/${version}/crystal-${version}-${toString rel}-${arch}.tar.gz";
+
   genericBinary = { version, sha256s, rel ? 1 }:
     stdenv.mkDerivation rec {
       pname = "crystal-binary";
       inherit version;
 
       src = fetchurl {
-        url = "https://github.com/crystal-lang/crystal/releases/download/${version}/crystal-${version}-${toString rel}-${arch}.tar.gz";
+        url = binaryUrl version rel;
         sha256 = sha256s.${stdenv.system};
       };
 
@@ -225,6 +232,7 @@ rec {
       x86_64-linux = "1949argajiyqyq09824yj3wjyv88gd8wbf20xh895saqfykiq880";
       i686-linux = "0w0f4fwr2ijhx59i7ppicbh05hfmq7vffmgl7lal6im945m29vch";
       x86_64-darwin = "01n0rf8zh551vv8wq3h0ifnsai0fz9a77yq87xx81y9dscl9h099";
+      aarch64-linux = "0sns7l4q3z82qi3dc2r4p63f4s8hvifqzgq56ykwyrvawynjhd53";
     };
   };