Merge pull request #161156 from a-m-joseph/abort-on-failed-platform-detection-instead-of-silently-assuming-pc

platforms.nix: use {} on failed detection instead of silently assuming pc

lib/systems/platforms.nix

@@ -3,7 +3,7 @@
 # targetPlatform, etc) containing at least the minimal set of attrs
 # required (see types.parsedPlatform in lib/systems/parse.nix).  This
 # file takes an already-valid platform and further elaborates it with
-# optional fields such as linux-kernel, gcc, etc.
+# optional fields; currently these are: linux-kernel, gcc, and rustc.
 
 { lib }:
 rec {
@@ -568,5 +568,5 @@ rec {
 
     else if platform.parsed.cpu == lib.systems.parse.cpuTypes.powerpc64le then powernv
 
-    else pc;
+    else { };
 }