gcc7: 7.4.0 -> 7.5.0

It's bugfix-only; these tend to be fairly safe,
and we don't really use gcc7 anymore by default.
I tested building gcc7 and gfortran7 on x86_64-linux.
https://gcc.gnu.org/ml/gcc-announce/2019/msg00003.html

pkgs/development/compilers/gcc/7/default.nix

@@ -43,7 +43,7 @@ with stdenv.lib;
 with builtins;
 
 let majorVersion = "7";
-    version = "${majorVersion}.4.0";
+    version = "${majorVersion}.5.0";
 
     inherit (stdenv) buildPlatform hostPlatform targetPlatform;
 
@@ -85,7 +85,7 @@ stdenv.mkDerivation ({
 
   src = fetchurl {
     url = "mirror://gcc/releases/gcc-${version}/gcc-${version}.tar.xz";
-    sha256 = "0lgy170b0pp60j9cczqkmaqyjjb584vfamj4c30swd7k0j6y5pgd";
+    sha256 = "0qg6kqc5l72hpnj4vr6l0p69qav0rh4anlkk3y55540zy3klc6dq";
   };
 
   inherit patches;