lib.systems.inspect.patterns.isGnu: init

This allows checking e.g. stdenv.hostPlatform.isGnu, just like isMusl
or isUClibc.  It was already possible to check for glibc with
stdenv.hostPlatform.libc == "glibc", but when that doesn't line up
with how every other platform check works, this is apparently
sufficiently non-obvious that we've ended up with stuff like adding
glibc.static if !isMusl, which is obviously wrong.

lib/systems/inspect.nix

@@ -56,6 +56,7 @@ rec {
     isNone         = { kernel = kernels.none; };
 
     isAndroid      = [ { abi = abis.android; } { abi = abis.androideabi; } ];
+    isGnu          = with abis; map (a: { abi = a; }) [ gnuabi64 gnu gnueabi gnueabihf ];
     isMusl         = with abis; map (a: { abi = a; }) [ musl musleabi musleabihf ];
     isUClibc       = with abis; map (a: { abi = a; }) [ uclibc uclibceabi uclibceabihf ];