lib/systems: musl, libc predicates

Note this doesn't actually provide musl support yet,
just improves our "system" code to understand
musl-based triples and non-glibc linux configurations.

lib/systems/parse.nix

@@ -180,6 +180,9 @@ rec {
     androideabi = {};
     gnueabi = {};
     gnueabihf = {};
+    musleabi = {};
+    musleabihf = {};
+    musl = {};
 
     unknown = {};
   };