linux/common-config: Split Rust toolchain and kernel-based conditions

The ongoing RISC-V breakage may become version independent in the
future, move it to the correct condition before updating it with the
full list of supported architectures.

While at it, simplify the remaining rustc-specific conditions into an
`availableOn`.

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

@@ -43,7 +43,10 @@ let
   );
 
   forceRust = features.rust or false;
-  kernelSupportsRust = lib.versionAtLeast version "6.7";
+  kernelSupportsRust = lib.versionAtLeast version "6.7"
+    # Known to be broken: https://lore.kernel.org/lkml/31885EDD-EF6D-4EF1-94CA-276BA7A340B7@kernel.org/T/
+    && !(stdenv.hostPlatform.isRiscV && stdenv.cc.isGNU);
+
 
   # Currently only enabling Rust by default on kernel 6.12+,
   # which actually has features that use Rust that we want.

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

@@ -109,11 +109,7 @@ 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);
+    rustAvailable = lib.meta.availableOn stdenv.hostPlatform rustc;
 
     features = kernelFeatures; # Ensure we know of all extra patches, etc.
   };