Adding gccgo for gcc4.7.

I tested hello world, and it works.

pkgs/development/compilers/gcc/4.7/default.nix

@@ -52,7 +52,7 @@ let version = "4.7.1";
     crossGNU = cross != null && cross.config == "i586-pc-gnu";
 
     patches = [ ]
-      ++ optional (cross != null || langGo) ./libstdc++-target.patch
+      ++ optional (cross != null) ./libstdc++-target.patch
       # ++ optional noSysDirs ./no-sys-dirs.patch
       # The GNAT Makefiles did not pay attention to CFLAGS_FOR_TARGET for its
       # target libraries and tools.
@@ -249,6 +249,10 @@ stdenv.mkDerivation ({
     (ppl != null && ppl ? dontDisableStatic && ppl.dontDisableStatic)
         [ "--with-host-libstdcxx=-lstdc++ -lgcc_s" ];
 
+  # 'iant' at #go-nuts@freenode, gccgo maintainer, said that
+  # they have a bug in 4.7.1 if adding "--disable-static"
+  dontDisableStatic = langGo;
+
   configureFlags = "
     ${if enableMultilib then "" else "--disable-multilib"}
     ${if enableShared then "" else "--disable-shared"}