gdb: Flexible target matching for darwin

Outside of the nix-build the target is `x86_64-apple-darwin17.4.0`,
while inside the target is `x86_64-apple-darwin`. This difference
causes the fallback target configuration for darwin, which disables
gdb. Add a patch to make the target matching more flexible.

pkgs/development/tools/misc/gdb/darwin-target-match.patch

@@ -0,0 +1,11 @@
+--- a/configure	2017-06-05 00:51:26.000000000 +0900
++++ b/configure	2018-03-06 23:12:58.000000000 +0900
+@@ -3603,7 +3603,7 @@
+     noconfigdirs="$noconfigdirs ld gprof"
+     noconfigdirs="$noconfigdirs sim target-rda"
+     ;;
+-  x86_64-*-darwin[912]*)
++  x86_64-*-darwin*)
+     noconfigdirs="$noconfigdirs ld gas gprof"
+     noconfigdirs="$noconfigdirs sim target-rda"
+     ;;

pkgs/development/tools/misc/gdb/default.nix

@@ -35,7 +35,8 @@ stdenv.mkDerivation rec {
     sha256 = "0d2bpqk58fqlx21rbnk8mbcjlggzc9kb5sjirrfrrrjq70ka0qdg";
   };
 
-  patches = [ ./debug-info-from-env.patch ];
+  patches = [ ./debug-info-from-env.patch ]
+    ++ stdenv.lib.optional stdenv.isDarwin ./darwin-target-match.patch;
 
   nativeBuildInputs = [ pkgconfig texinfo perl setupDebugInfoDirs ]
     # TODO(@Ericson2314) not sure if should be host or target