lib.systems: force qemuArch to "aarch64" on aarch64

qemu architecture names are fixed — we're using uname here just
because it's more likely to be correct than CPU name (see e.g. POWER).
This means that aarch64 is always called aarch64, even on Darwin where
uname reports arm64.

Fixes: 61582c7043 ("lib/systems: use Darwin architecture names for `config` and `uname`")

lib/systems/default.nix

@@ -305,6 +305,8 @@ let
           qemuArch =
             if final.isAarch32 then
               "arm"
+            else if final.isAarch64 then
+              "aarch64"
             else if final.isS390 && !final.isS390x then
               null
             else if final.isx86_64 then