linux: implement rustAvailable condition

* It doesn't matter if `rustc` is available, but if rustc can compile to
  the hostPlatform. So use a custom condition instead of `availableOn`.
* Explicitly exclude the combination of GCC and riscv which is known
  to be broken[1].

[1] https://lore.kernel.org/lkml/31885EDD-EF6D-4EF1-94CA-276BA7A340B7@kernel.org/T/

Co-authored-by: Alyssa Ross <hi@alyssa.is>

pkgs/os-specific/linux/kernel/common-config.nix

@@ -11,6 +11,7 @@
 
 # Configuration
 { lib, stdenv, version
+, rustAvailable
 
 , features ? {}
 }:
@@ -34,7 +35,7 @@ let
 
   # Currently only enabling Rust by default on kernel 6.12+,
   # which actually has features that use Rust that we want.
-  defaultRust = lib.versionAtLeast version "6.12" && (stdenv.hostPlatform.isx86_64 || stdenv.hostPlatform.isAarch64);
+  defaultRust = lib.versionAtLeast version "6.12" && rustAvailable;
   withRust = (forceRust || defaultRust) && kernelSupportsRust;
 
   options = {

pkgs/os-specific/linux/kernel/generic.nix

@@ -108,6 +108,11 @@ let
 
   commonStructuredConfig = import ./common-config.nix {
     inherit lib stdenv version;
+    rustAvailable =
+      lib.any (lib.meta.platformMatch stdenv.hostPlatform) rustc.targetPlatforms
+      && lib.all (p: !lib.meta.platformMatch stdenv.hostPlatform p) rustc.badTargetPlatforms
+      # Known to be broken: https://lore.kernel.org/lkml/31885EDD-EF6D-4EF1-94CA-276BA7A340B7@kernel.org/T/
+      && !(stdenv.hostPlatform.isRiscV && stdenv.cc.isGNU);
 
     features = kernelFeatures; # Ensure we know of all extra patches, etc.
   };